I should explain that those squares are not meant to be lines of latitude and longitude. They're simply squares 600 miles on a side.

Why are they there? Because when and if I decide to develop one of my continent maps for RPG gameplay (which is the ultimate goal), I can import each 600 mile x 600 mile square into hexographer and break it into 30-mile hexes, which is what I've found optimal for RPG gameplay at a strategic scale. It's also then convenient to break the 30-mile hexes into 6-mile subhexes, which I've found optimal for RPG gameplay at the day-to-day scale.

Why so? Well, there's a whole blog-post in praise of 6-mile hexes here with which I agree, and it also happens that my preferred system has daily movement rates in multiples of 6 miles.

Does that mean my map won't project correctly onto a globe? Yes. It's a big flat oblong. At least with these first few maps that I'm making, I'm deliberately sacrificing geophysical realism on the altar of convenience in play.