Perhaps the plural is used due to the call for "short finals" or long "finals" It seems to imply a two stage distance based call on final approach.
Often the call gets made as "finals full stop" or "finals to roll" or finals touch and go" At least it leaves aircraft on base leg an idea of what your doing.
To throw more into the pot you will also hear people add the words "three greens" into the final's call to demonstrate that they have checked the gear is down and showing locked on a retractable.