This is the case both on really low and really high difficulty settings. At low difficulties, it's easier to get more land, and techs are cheaper so that you can both generate more beakers, and the techs cost less (what Zexx was saying). At high difficulties, the AI techs really fast due to those same factors, and due to trading techs with the AI to keep up with them, the player also techs really fast in comparison (what TMIT was saying).
Basically, due to the advantages that the player or AI has at the extreme ranges of the difficulty spectrum, you can get crazy tech rates at these levels. If you're playing somewhere in the middle, the tech rate is generally slower. You can still occasionally get enough land/commerce (GLH on archipelago with tons of intercontinental routes, a bunch of cities with cottaged riverside grasslands, etc) to tech this fast in the middle difficulty levels, but it's far more rare.