A lot of it is D3D API immaturity, some of it is 3dfx being arcade first with no CAD ambitions. And then there's sometimes the silly placebo effects of quality coming from the high default gamma values (unsurprising given arcades' CRTs) and the smeary smudgy filter I like to raise awareness about for the thousandth time here since that's the big unmentioned 3dfx exclusive feature*. Comparing 3dfx vs the 2nd/3rd generation of 3D accels (TNT, i740, Rage 128, etc) is much more subjective when they're all capable with blending functions.
Also another factor how "3dfx better!" is some engines, mainly UnrealEngine1, don't have the best feature parity or optimized D3D/GL driver.
* = some 3rd party drivers "fix" this by disabling it in the registry. also V2 SLI doesn't filter due to timing issues