Honestly, the only way you'll know what's going on is if you have the alignment checked.
This will highlight where the issue lies.
I can understand why it would pull to one side with miss-matched tyres (wear-wise), as the rolling diameters will be different. The only other thing I can think of is there is a huge variance in tyre pressures which has gone un-checked.
FWIW, cars aren't meant to track dead-straight. The alignment specifications take into account the camber in the road, meaning on a perfectly flat road, the car will have a tendency to list to the left. This is also a safety feature to help prevent you from veering into oncoming traffic if you fell asleep at the wheel, for example.
Wrecking: '97 Golf 3 CL 1.8L - still have motor, panels, and electrics!
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