From the FSF
I can tell you exactly the reason for this.
Leaving aside the middle tier, there is no neutral area. Everywhere is either Stoke or Man City and the areas are equal in size. The so-called football family have to claim their entitlement and request in which area they want their tickets and unused allocations are given to the competing clubs.
If there is a less than 100% take-up of these allocations and if there is a bigger take-up for one club than the other, then the “smaller” club will inevitably get more “extra” tickets. Man City haven’t got any extra tickets because the take-up for the Man City areas from the football family is near 100%, but is less for the Stoke areas (this is not particularly surprising, given that, whether we like to admit it or not, Man City have both a higher profile and a bigger fan base)
So, in reality it evens out, because there are more Man City “supporters” in the “football family” areas. However, because they have got these tickets because they are either involved with one of the football family organisations or know someone who is, not through the Man City priority scheme, many of them may just be people with an affiliation to Man City, or people who don’t support either team but have a preference for sitting in the Man City area rather than the Stoke City area. As a result, we will have a higher proportion of more regular fans.