Wikipedia says the Romans had a different use of the term:
a century was actually only 80 men
It is like the opposite of a “baker’s dozen”. Seriously, though, it looks like regulations modified the definition of a Centurion’s establishment.
Centurions took their title from the fact that they commanded a century. Centuries were so-called because they originally numbered roughly 100 men. Just after that they numbered 60 men each and were paired into maniples, one with greater authority. After the Marian reforms, however, the standard establishment was set at 80 men.
What, they couldn’t rename things?
The Roman legionary army lasted over a thousand years — just under two thousand years, if you take the fall of Constantinople as the end — and changed considerably and several times over that period.
By the time of the Marian reforms, the legion structured army was already around four hundred years old, and “centuria” had come to mean an infantry company, regardless of strength (and also, incidentally, a small voting district to which military recruitment was intimately tied until the Marian reforms.) In fact, in a Marian legion there were two sizes of century: 60 “normal” ones of 80 legionaries each, and 5 double sized ones, with 160 legionaries each. (Also, I believe these counts include only the legionaries, and not the NCOs of the centuria: optio, signifer, tessarius and discentes signiferorum.)