Bear in mind that the Perfect Body is what Cell is
designed from day one by Doctor Gero to become, not a random occurance of happenstance. The evil bastard probably even drew what his ultimate creation's form would look like on a blueprint in white ink much like he did with 17 and 18!!
The Perfect Body, much like all the transformations in Dragonball Z, is not so much a change in physiology as a change in power. It is safe to assume that even if Cell never absorbed 18
at all in the first place and survived a similar cataclysmic suicide, the sheer
power he gains would alter his pysiology to the Perfect Body anyway....
...shiver.
