I've dropped 22 gravel on a torch 4 times and didn't get a single piece of flint. The odds of this happening are less than 1 in 10000, unless:
1. The Minecraft Wiki is incorrect and the odds of gravel dropping flint are not 1 in 10.
2. This is an intentional feature to prevent easily destroying lots of gravel for flint (possibly to help contain the chaos that ensues when a gravity-affected block duplication exploit is discovered)