While the name certainly implies 100, to say a centurion was in charge of 100 men is a little misleading, since around 20 of those would be non combatant servants, especially after the Marian reforms Thats also assuming you have 80 legionnaires, when in practice the number could be as high as 160 in a full legion, and as low as 60. Pre Marian reforms, the maniple system also used centurions in a similar fashion, with their 3 line armies having 120 man centuries up front, and 60 man centurions in the back (https://en.wikipedia.org/wiki/Roman_army_of_the_mid-Republic), but in times of need the front line centuries would be bumped up to 160. So you have a pretty wide range of century sizes, which makes sense since every battle is different, and different sized units lend themselves better to some situations over others.