PRS3,
As the others have said it is completely normal. It's caused by the gear sets not being quite aligned enough to drop into place when you finally come to a stop.
There is no need to worry, dropping in while slightly rolling is one way, but sometimes you can't do that if you're stopping in a hurry, or in heavy traffic and sitting in neutral.
When you let out the clutch that little bit, with pressure on the shifter it will drop straight in because it's helping line up the gear sets. Your manual car may do it as well, especially into reverse.
So there's the mechanical $0.02!
Mick