I think it's both. The higher stakes will naturally look more swingy because they dwarf the lower stakes - notice how $2NL is barely visible. It's also compounded because $25NL is a bigger jump money wise than the other stakes - $25/$10 = 2.5 whereas $10/$5 = 2. But, yes, better players, harder to get paid, punish you a bit more when you pay them, and the like. Speaking of which ...