Episode #4.7 - S4-E7
Factual error: Phony computer shenanigans happen when JP shows the rest of the gang the file he recovered from the trash; he plays a PX (Pixel32 file, unless it's meant to be some file for a fictitious app) like it's a full HD amazing video, straight from the bin without recovering it first. (00:33:05)
Episode #4.5 - S4-E5
Factual error: Humphrey mentions that the fire that destroyed the recording studio happened in July 1991. The first page of the Saint Marie Times carrying the news though has Thursday 12th as a date; in July 1991, the 12th was a Friday. (00:16:25 - 00:16:55)
Answer: There's probably no particular reason. Sets and props on long-running TV shows often change as needed and for various reasons throughout a series run.
raywest ★
As long as such changes aren't an obvious continuity mistake (e.g. changing multiple times during a day), one can just assume the change happened in-universe as well. Things do get broken or updated occasionally. It adds a bit of realism, in my opinion.