I'm sorry if this sounds incredibly condescending, but I find Harry Potter secrets like this kind of adorable, like the people making them sound like they've pondered something for the Very First Time and it's not the most hotly debated issue in an enormous fandom or anything.
OP, if you don't ship Ron/Hermione, you're definitely not alone, there are MANY people who feel this way, often partly because they ship Hermione with Harry or someone else, and sometimes, more rarely, because they ship Ron with someone else. You can do the same as long as long as you're nice to the people who feel otherwise, as it goes in all shipping wars. It's best you don't phrase it as "why Hermione chose him", because that sounds like Ron-bashing and there's a stigma against Ron-haters in this fandom because he got a lot of hate during the times of the great R/H verses H/H ship wars. I'm not sure what Rupert Grint as an actor has to do with this secret, though. Hermione's not with Rupert Grint, she's with Ron Weasley. Rupert is a real person, Hermione is a fictional character.
And if you already knew all that, and I'm just misinterpreting this secret, I'll stop being a condescending prick/ insufferable know-it-all