Just to throw a spanner in the works, my gauge does this too, but the other way.
Even though I'm running on LPG, the petrol gauge will continue to fall gradually, but every so often it seems to "realise" that I've used next to nothing, then jump back up again. The jump is typically a quarter of a tank. Even the amber fuel warning light isn't immune to this, when the gauge jumps back up (which it will do whilst I'm on the move) the light goes back out.
My guess is that it only checks and redisplays the actual capacity every so often, unless there's a big jump in the reading (in other words you've put some more in), in both cases the actual fuel level then gets displayed. In between, it's just guessing how much has gone based on the same figures your trip computer (if you have one) would use.