Each of the SID6581s I have tested sound different, particularly when it comes to filter cutoffs and distortion. What are the causes of these variances?
SID6851 specification quotes:
Cut-off frequency variation may occur from chip to chip due to process variations, and power supply voltage. Capacitor values and voltage regulation can compensate for these variations
We're talking about a 30-some years old chips, the manufacturing process wasn't very consistent back then, and also MOS was known to have issues with some process steps such as passivation layer that was "leaking" and allowing external elements to "age" the chip.
Also, SID is in big part an analog chip, there are plenty of analog components there on a die (this is why it's so hard to reimplement say in FPGA), each and every one of those analog components can vary between chips slightly.