each tile starts with culture 0. as soon as it's within the reach of a city (this is not equal to city borders) the cultural influence of this city gets added to this tile.
example:
russian and aztec place cities near to each other, both pumping culture into tile A.
after a while russia has 800 culture in this tile, aztec has 300. so the tile belongs to russia.
then aztec declares war (no surprise) and destroys the russian town.
but within this round tile A still holds 800 russian culture. so it won't fall within aztec borders. the aztec borders will wrap around this tile.
it won't be just tile A, there will be tile B, C, D etc. and the next tile with less russian than aztec culture lies behind them. thus forming irregular culture spots.
i don't know if this is intentional as it's most likely "fixed" within one turn.
but it also applies to "contested" tiles. with largely expanded borders it could come to some weird cultural influences.
edit: another thing - you can't hold posession of "deep ocean" tiles. but of "low ocean/coast" tiles behind
