Aside from the debate on how how to build your arena, it's true that there seems to be a attendance that is unrealistically stable.
I believe there is a soft cap
around 20 000 seats. That doesn't mean its effect cannot be felt before reaching 20 000 though.
In fact, the attendance cap works much like an
audio compressor. We don't know the ratio (we just know it's finite), or whether it has a
knee (http://en.wikipedia.org/wiki/Dynamic_range_compression#So...), although we know the threshold is around 20 000.
BUT, if there is indeed a knee, then even with a threshold at exactly 20 000, we could see a capping effect around 19 500, although that effect should be minimal (as in, soft, capping harder the more seats you sell), and would not entirely explain why you kept selling exactly 19 500 seats for 8 straight games despite having extra seats available.
My feeling, just by looking at your arena, is that there is a a specific cap for bleacher seats, perhaps 4 independent caps for each seat category.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme