All differ slightly because they use slightly different data sets and have their own ways of adjusting for relocations of weather stations and changes in instrumentation over time. (newscientist.com)
• the inherent difficulties in correcting for changes in instrumentation and in the siting of radiosonde and surface stations. (nap.edu)
However I also understood that trends determined from raw data can suffer from changes in instrumentation and / or changes in location. (landscapesandcycles.net)