This! I was told it was based on previous attendances when I complained about the Chelsea allocation the year after that game as the £30 cap had then come in and Virgin Media were subsidising £10 per ticket. Less than half the cost from the season before and they still took the smallest allocation!
I emailed the ticket office about this last week. Virgin Media announced their subsidising of tickets the day before tickets went on sale, by which time we had already turned down extra tickets - so it was never an initial allocation!