Not uncommon phenomenon. It depends on the powder used, actually. In most any cartridge, one can find a very slow powder that will work very well if packed in and the bullet beat into position. A 'fast' powder for the same cartridge will require less volume (or weight, depending on how one measures) and leave extra space.
I tend to use slower powders and load them full. I think it gives me a more uniform burn rate, from round to round.
Hatcher's Notebook mentions this. Some early loads for the 03 Springfield would give different velocities if the rifle were held muzzle up vs. being pointed at the ground prior to firing. Seems to be the position of the powder in the case affects the burn results.