Its not a mistake. Page 45 of the Blue Book is rather explicit under the 'Planets' heading:
. In the Gothic Sector there are over two hundred inhabited worlds and tens of thousands of other planets. Planets often become the focus of space battles as opposing fleets attempt to estalbish forward bases or extend their control throughout a system.
This is consistent with the Rennie novels which describe the Gothic sector as having hundreds of worlds (interestingly enough BOTH books also mention the sector having hundreds of vessels, and this was BEFORE the battle of Gethsemane.) There is also page 316 of the Black Crusade Core Rulebook by FFG:
...each cluster is as a chain of islands in the midst of a vast ocean.The million or so worlds directly controlled by the Imperium are, therefore, to be found in fairly dense groupings called sectors, and the gulfs between each is known as Wilderness Space. Even amongst the two hundred or so settled worlds within the average sector....
Which again fits. Nothing in my calcs (unless they're very very old calcs, but quite obviously I've redone the battlefleet gothic stuff) to my memory involved confusing the size of sectors and the number of worlds.
We also know from Deathwatch: Rites of battle, page 214:
These regions are the domain of the Imperial Navy; made up of thousands of sector battle fleets and countless vessels the Imperial Navy patrols the void and protects the Imperium's obrders as well as the darkness betwen its worlds.
Thousands of sector battlefleets. This actually meshes with a comment made by
Andy Chambers WRT Battlefleet Gothic and the number of sectors of the Imperium. From the above link:
Q: In the ship section, there is mention of Imperial Oberon-class battlecruisers and Apocalypse-class battleships. Were these cut from the registry to save room? Also, will these be making an appearance in a future issue of WD or possibly a supplement?
A: No we just wanted to give an impression of there being a greater number of ship classes and designs in the galaxy than were depicted in the book, particularly as Gothic sector is but one among thousands. Having said that we might well put some extra ship designs in WD at some point
We can even extrapolate further from Rogue Trader RPG (again FFG) and their sourcebook Battlefleet Koronus, page 67:
..lord high-admiral is the highest rank of the Imperial Navy. Only five of these august individuals exist, one for each of the Segmentae Majoris. Each is responsible for the Imperial Navy’s fighting forces across the thousands of sectors in their allotted quadrant of the galaxy, or in the case of the Lord High-Admiral Solar, the substantial volumes around Holy Terra itself.
Thats thousands of sectors
per segmentum. And there are hints that battlefleets span THOUSANDS of vessels per sector in that same book (Page 68)
In ancient times when an entire Battlefleet might be massed together, the thousands of ships present would be divided in to three commands, each under their own admiral. The rearadmiral was the youngest and least experienced and so would be given charge of the rearmost division as the one least likely to see combat.
It can be wrangled about lots of ways. Some sectors have gotten as small as 50 worlds (Corribra sector from the BFG Magazines, the sector in the Last Chancers novel 13th Legion) but that actually gives MORE sectors, not less. And I've been hard pressed to think of sectors being larger than a couple hundred worlds.. a few sources hint at a thousand or more worlds per sector (inhabited, Imperial worlds) but that would be unusual unless we assume there are 'millions' of worlds in the Imperium (which would only push the number of sectors up again, but has been mentioned in many sources and is thus quite possible.)
As far as 'going by sector volume'.. those simply wont work. The problem with that is its made quite explicit in BFG and other sources that sectors and the subsectors within them are not rigidly defined. The Imperial held systems and territories are scattered throughout the sector, sometimes separated by vast tracts of wilderness space that is NOT inhabited (and sometimes not even explored) by the Imperium. On top of that, sectors are not neccesarily adjacent to each other. Just as worlds within a sector can be scattered about and separated by uninhabited space, the same is true of sectors. Its sometimes mentioend that planets can be separated by hundreds or even thousands of light years of space, and we simply don't know how far apart sectors are as a rule (and even then its probably not consistent.) At best, you're going to get a very generous upper limit (or an approximation depending on the assumptions made) but its not going to be terribly reliable, as you have discovered.
