Something similar happened to me when i forgot to take mine off when went horse riding and it said something like 35000 steps and thats probably cause in the different gaits there are ranging from 2 to 4 err pluses as it were!
They measure the movement rather than the acctual step so if your cycling was at a nice speed, steady and not on a very bumpy road or even on a stationary bike then it wouldn't register that well!