I do think it's a mixture. As I said I know of an Albion STH who is first and foremost a Chelsea fan. I can't believe I know the only Amex STH for whom Brighton isn't his first club.
(This is what happened last time this topic came up. There was a level of disbelief when it was suggested that...
This is an unpopular view (for which I have been shouted down in the past but I'll make the point again), but some are probably using the tickets they use every week. Chelsea are their 'big' team but they're happy to support Brighton the rest of the time.
And no I can't quantify it, but I do...