You’ll also see a lot of “studies” use either 1860 or 1910 (or there about) as a reference. No coincidence that - those were cool years. In the normal fluctuations and cycles of temperature, there are going to be highs, and there are going to be lows. So if you pick a time when those fluctuations and cycles result in several years of relatively low temperature... Well, then everything else starts looking pretty warm in comparison. Data manipulation 101.
Not to mention all of the thermometer-based temps from before around 1900 are totally wild-ass guesses, because there was no way to regulate thermometers from one location to another.