Heres something that just got me (again), my ZTR broke its engine to hydros belt. Looking at the parts manual, the part is H18021, OR Dayco A39, OR 1/2" x 41". However, going to the store and picking up a 4L410 (1/2" x 41") resulted in a belt that was waayy too short. Knowing that "A" belts and "4L" belts are measured differently with "A" belts being 2" shorter, thus "A39" or 1/2"x41"(duh!), I figured I was good and the manual was correct, but then why didnt the 4L410 fit? Well, looking at the original Gizmow belt, I found that it was only 3/8" wide, not 1/2". This made it fit down into the pulleys farther which means a shorter belt. The manual was incorrect. I ended up with a 4L430 (1/2" x 43") belt that works perfectly.
So, long story short: Dont always trust the manual, dont always assume that the original is 1/2" or 5/8" as it may be an odd size (proprietary) and measure it before you go, and dont trust 'conversion' charts when dealing with OEM part numbers. Measure, measure, measure.