Dilution I'm guessing. When I step up yeast I don't decant either, but I'm going from slant to 20ml to 200ml to 2L. 10% isn't much dilution and you can just bump the °p up if it freaks you out too much.
I wonder about staged starters when people are pitching 100ml to 1L, decanting and adding another 1L. At some point it seems the growth will not be what some expect because the yeast amount will be getting close to max for the volume anyway. I know I get great growth with 1:10 and probably could go 1:20 if the nutrients are there.
Edit: Denny was typing while I was. Lol