How are you timing your 100m? Are you sure it is 100m?
Usually, when I see people timing 100m in practice, they are super sloppy, or else inconsistent with how they time. IE – they start with a short jog in, hitting start button at around the start line, or whenever they start running hard. This takes 0.5 off your time.
Or, they time the 100m as the straight. Or they stop early, etc.
The longer the distance, the less impact of timing variation.