I suspect the 'r' entered as a non-rhotic spelling variation.
I believe that non-rhoticism is much more recent than the 400 year b.p. date given for the introduction ofn
sherbet and its almost immediate variation,
sherbert.
Wikipedia has this:
The earliest traces of a loss of /r/ in English are found in the environment before /s/ in spellings from the mid-15th century: the Oxford English Dictionary reports bace for earlier barse (today "bass", the fish) in 1440 and passel for parcel in 1468. In the 1630s, the word juggernaut is first attested, which represents the Hindi word jagannāth, meaning "lord of the universe".
LanguageLog does have this:
The traditional account (as e.g. in (Richard) Bailey 1996 and Lass 1992) was that loss of postvocalic /r/ in England was a 17th and 18th century phenomenon. Thus r-lessness would have been widespread (but not universal) during the period when English speakers emigrated to North America, and thus settlement patterns are a likely source of influence.
However, recent research suggests that "... most of England was still rhotic ... at the level of urban and lower-middle-class speech in the middle of the nineteenth century, and that extensive spreading of the loss of rhoticity is something that has occurred subsequently..." (Peter Trudgill, "A Window on the Past: "Colonial Lag" and New Zealand Evidence for the Phonology of Nineteenth-Century English". American Speech 74(3) 1999).
