The difference in both the variation in ratios and the variation in "cup" meaning which other answers here note, is the Atlantic Ocean.
2 tbsp per 6 oz of water is Americano (relatively weak). 2 tbsp per 4 oz of water is European (stronger, because it's the same coffee in a smaller cup). (Some places in the Caribbean seemed to use even higher ratios, perhaps reflecting European influence; but perhaps it was rather than the coffee was fresher.)
The variation in the word "cup" is of the same order: American machines typically provide measurements in 6 oz cups. So, if you have an 8 "cup" machine, that's 8 American "cups" x 6 ounces = 48 ounces, for which you might use 16 tablespoons - but is also 12 European "cups", for which you might use 24!
(Note that there are 16 tablespoons in a measuring cup. So, don't bother with tablespoons at all when making coffee. For 8 "cups" of water, use one measuring cup of ground for Americano, and 1 1/2 for, um... real coffee. ;)