How are you testing this? Indicator on the head riding in the T-slot?
If you are checking the sides of the T-slot, be aware on early machines (well before the S3) none were precision machined and after that only the center slot was for some time. You might verify the error is consistent in other places if you haven't yet.
Otherwise it could be somehow the table isn't square to the X axis.
FWIW, I had a very early 1100 that had all manner of abuse before I got it (the stand was dented as if it had been dropped on one corner) but once I got it level and squared up the vise, it made good parts for many years.