Why the green arrow for the “wrong” one, and the red arrow for the “right” one?

3 of the 4 items (gender doesn’t matter, variety does) are generally correct.
Source: I’m a former watermelon “cutter” (the guy that goes out in the field first thing in the morning and cuts the good melons off the vine, and turns them belly side up so it’s obvious to the field workers which melons to load up)

Also, with the whole thumping thing, most people just look silly doing as they don’t know what they’re doing. If you do thump, ones that have a higher pitched ping are still green, and that have a really dull/flat thud are over-ripe/too gritty/sugary. Also, weight should feel right, too light and it’s overripe/rotten.

In general, any melon sold at the store should be good, just take one and stop trying to be a hero. At least the farmers I dealt with are pretty ethical, they aren’t purposely shipping bad melons. It just takes experience of seeing/handling melons for a while to get the “picking one” correct. Most store I know of have a satisfaction guarantee anyway, take pictures if it’s bad and when you go back get a refund if you’re that concerned with it.