I have 2 pairs of 2.3s. One is the white/gray (US8) general release, the other is white/blue (US8.5) rafa clay season. I can't explain why the white/blue ones (the larger ones) fit me better, even if its larger. And the issue is, the white/gray has heel slippage but the white/blue has none. Can someone explain what's the reason for this? Thanks for any answer.