Its a mixture of multiple things, some are historic american expansion ideas such as Cuba, Canada, Northern Mexico, and the Yucatan while others are colonies Philippines or historically occupied regions Iceland, Faroe islands, and Sicily. Also some states are split up for some fucking reason. I dont know what the fuck is going on with Nicaragua and why it only maps out the Dominican republic side of Hispaniola.

Edit: I just remembered that some Americans at one point tried conquering Nicaragua to recreate the Federal Republic of Central America. So that explains that.