In the shirt you linked to, there is X-Large Tall and 1X (and then 2X and 2X Tall) but there is no plain XL. 1X is the same as other places' XL, I don't know why they make it different for the Tall one vs the regular one. But given the rest of their sizes (2X, 3X etc) 1X makes sense.
I looked at some other shirts, which used XL and XXL instead of 1X and 2X. Maybe it depends on the brand. Nope, I checked several other shirts with the same brand, and this is the only one I could find that used this sizing nomenclature. Just weird, I guess.