It is most likely intentional, though I've never seen it done in code (I program for a living). However in mathematical term IFF stands for "if, and only if".
IF you have one leg = true
I'm assuming you have two legs.. which means you definitely have one leg.
whereas
IFF you have one leg = false
Again, assuming you have two legs, since you don't only have one leg.
Mathematically, a C++ "if" is "if, and only if"
if (numLegs == 1) ...
would not be true if you had two legs.