It is well known in the recording biz and amongst "78" collectors that a very high percentage of recordings made before standardization came in around 1945, were not at 78rpm. Many companies had there own speeds, usually somewhere between 74rpm and 80rpm. Bear in mind that in those days players would have a "Speed Control" at the side of the turntable and one could adjust it to whatever speed sounded right.
In the vinyl days, transfers from "78's" were usually made at 78rpm without bothering to check the correct speed. In more recent years however, more thought has gone into trying to get it right.
A variation of 20% is very unlikely. Up to 10% is the norm. Another reason for recordings that played too fast were that it was an easy way to fit a song lasting, say, 3.5 mins onto a 10" record that could only accomodate 3.25 mins. Many artistes were asked to speed up their performance, but if they refused, slow down the recording blank by about 8% and problem solved.