Humans didn't exist since the beginning of time, and we only started to properly record temperatures in the last few centuries. That means we have to determine historical data through the effects it had on our planet. The methods to find this historical data from the effects keep changing and evolving, so it makes complete sense to me that historical data has changed throughout the reports.

Unfortunately you didn't specify where one can find this "vastly different historical data", so I can't get more specific than this.