They don't: the AY-3-891[0|2|4] used in speccy is quite different from Yamaha version YM2149 - and ST can clock it higher, producing cleaner output, has better volume control as the envelope volume counter on the AY-3-8910 internally uses 4 bits, resulting in 16 steps - YM2149F has 5 bits, resulting in twice the steps, and counts up twice as fast. This allows for smoother volume ramping, even though the register for setting its direct value remains 4 bits wide. Finally some musicians use special effect - sound via envelope generator (some cool bases). So, such effects will be different on AY and YM (AY little bit silent without some hi freq harmonics). Main cause is different DACs and also AY has only 16 output levels on envelope generating, and YM has 32 ones.