It’s not due to improper installation, it’s due to improper aim. Over years of use they can experience spray and splashes of urine. The salt and ammonia in pee can be quite corrosive. You could try finding full brass hardware (not brass coated), or stainless hardware. That should help with the issue, also being diligent to clean up well if the bowl overflows or if there is an accident just outside the bowl will help too. There are cops that they make for the bolts that can help shield from splashes and make it look tidy too.I've noticed this on toilets in 2 houses. So I wonder is it normal for the anchor bolts of a toilet to rust away over time, and/or get moldy?
Or does that indicate improper install? Because I'm assuming the bolts don't encounter a lot of moisture if installed correctly?