I think it's the E-TTL program. The E-TTL is designed to capture the pics with more-natural feel. So at FEC 0, E-TTL will give you somewhere between fill-in look and flash look. I tend to do +1/3 to +2/3 FEC for the flash look and -1/3 to -2/4 for the fill-in look. Using FEL will make the metering slightly more accurate, but not by much.
I think that's what I've experienced....when I use FEL it was 200x better. But then again, I just got my Stofen for it. Local "pro" shop only has the one for the 420EX. They sell the 430. Sheesh.