Kicking: What does this mean?

I am fast when it comes to fins-- I am faster than anyone in my lane, and can easily keep up with everyone in the next faster lane, both from a speed and endurance perspective. Unfortunately, when it comes to kicking without fins (with a board), I am sloooooooow and I peter out easily. It's so odd to me because it would seem that even though fins are faster, you should be able to keep up with the same people when not wearing fins as you do when wearing fins (if that makes sense). What does this mean? I would appreciate any insight. P.S.-- I will check out Ande's "Help My Flutter Kick is Terrible" tips as well-- just trying to figure out what my problem could be.