Back in the manual typewriter days, every typo subtracted 10 WPM from your score. (Source: my junior-high typing class.) It seems to me that 76% accuracy would put you into negative WPM by that reckoning.
It is not a good way to measure "typing speed".
So if you type 100 characters and 24 of them are incorrect, the counted total is 100 - 10*24 < 0, independent of the test length.
This was in the time of typewriters; computerized text entry did exist but was certainly not ubiquitous. There was no real way to correct mistakes, so I can imagine typing correctly from the beginning was a bigger deal than it is now.