This is not French specific, Romanian shows a similar difference:
This discrepancy long predates the birth of modern Romance languages, in Latin, both chrestianoi and christianoi have been used to name Christians.
Here is a quote from a web page that refer to an essay that gave some clues about it (although I wasn't able to find the original essay):
An essay by late William Shandruk from the University of Chicago examines the ways in which Christ and Christian are spelled in Greek papyri. Chrestos, which was pronounced the same way as Christos, was a common slave name meaning “good” or “useful.” Confused by this, representatives of the Roman government often misspelled Christ’s name “Chrestos” instead of “Christos” meaning “anointed” or “messiah.” They also called the early followers of Christ “Chrestianoi” rather than “Christianoi.” The early Christians themselves went with the Romans here and often spelled their own name “Chrestianoi,” but they stuck to spelling “Christos” for Christ’s name.
For some reason1, the first form survived in Romanian and French while the other one did it in the remaining ones.
1 One of these being the natural evolution of both Chrīstiānum and chrestianum to old French chrestien anyway, while Christ spelling and pronunciation stayed close to the Latin. Thanks to @EauQuiDort for this point.