I think it is to differentiate the use of the word.
Common can be derogatory so possibly the use of commonest is to move the meaning towards frequent - as in 'the commonest description was his behaving like a common little oik'.
Anne the Pleater
I presume to dictate to no man what he shall eat or drink or wherewithal he shall be clothed."
-- The Hon. Stuart Ruaidri Erskine, The Kilt & How to Wear It, 1901.