This is something that needs to be put into our guide - it's as intended... sort of. BS has two internal font size options; you have 1-5 which are pre-defined in the backend, and the rest are sizes we've added directly in pixel size. If you manually change the code (ie, to size=18px for instance) it should be the same as size=5. It's a quirk of our base software, Xenforo, which by default only allows us to have 5 different sizes. That wasn't acceptable to us as we want the WYSIWYG to be more fully fleshed out, so we had to add our own pixel based system. XD
It's a little confusing, but intended, yes. 1, 2, 3, 4, 5 are just predefined sizes and the dropdown just defaults to those over our pixel sizes. I'll make a note to see if we can override it to just insert the pixel size in the future (you'll notice 1,2,3,4,5 don't have a px by them) so it's less strange, but for the moment you can, as I said, manually insert your own pixel sizes without issue.
Thanks. Better now.