I think they change their compilers and supported libraries at times. Thus you will sometimes see solutions submitted by other users in the past that use some speed library that is no longer available (psyco in Python for example) or less memory than recent solutions. I would say, don't over think it because it's probably just them updating their compilers.
Yes it's a bit unfair that older solutions can sometimes get less time and less memory than current solutions, but I think deleting the old records of previous submissions would be more unfair.