Without reading every reply, which may have already given similar advice, let me say this.
Tire manufactures do not have a "hard and fast" spec to which they adhere. IOW, not all 33 x 12.50 tires have an OD of 33" and a width of 12.5" (never mind the fact that the width is based on a wheel width of a specific size, and depending on YOUR wheel width, it may vary).
Besides that, EVEN IF you have two tires that are EXACTLY 33" tall and 12.5" wide you could have one tire that fits and one that rubs. To help visualize it without spending a bunch of time making a pic, just imagine this, if you can.
Tire one is a (somewhat squished) oval, 12.5" wide and (assuming a 20" rim) 6.5" tall.
Tire two is a rectangle, 12.5" wide and 6.5" tall.
Of course, no tire is going to fit exactly either one of those profiles, but hopefully it gives you an idea that there are a lot of potential options between one and the other. So a BFG KO2 and a Nitto Trail Grappler may have the "same" specs but that DOES NOT MEAN that both with fit equally.