Well it depends on how the rest of the board is working and feeling as to which particular tail suits best.
When the evil twins were evolving, it was found that the boards were a little skittish in the last part of a turn, thus it was required to straighten out the tail a little bit. Basially the swallow straitens, but gives a little more bite at speed than would an area squash.(ie early evo's.
These boards slowly evolved into pins as it was found out that it was giving better rail to rail response on these boards.
Basially it was just found the swallow was suiting the twins better. Another reason was the twins were having a little difficulty in driving it up vertical, and the swallow was giving a little extra drive off the tail.