Even if a bug is platform-independent, the report may well be platform-specific and difficult to reproduce. The real nature of the bug only becomes clear after analysis, and getting the right platform for the analysis may be so much work that the whole thing is impractical.
"May be" doesn't mean "very rarely" either. It's sad, really. You may sit in front of your keyboard with an intuition that this bug affects lots of people and know that you can't really work on it, so how to you respond? Sad.
Trying to reproduce a bug by doing something different than what the customer did is a good way to waste time, in my experience.
One of my first really nasty bugs of this kind was platform-independent, in the sense that the fix affected all platforms. Once I had found the buggy code it was easy to reproduce it on my development workstation by constructing a more efficient reproducer.
But it did not affect all platforms equally. With the hardware on my desk it led to a slow memory leak and would crash after a month or more, on a particular customer's hardware it leaked all memory in an hour, and the customer had more memory than I. I don't remember why, maybe the different CPUs changed whether a race was lost/won? Anyway, I could have gone a year without noticing the bug in my office, but even on the same platform as I personally used, customers would have infrequent crashes.
So. Platform-dependent or not? It's hard to classify in those terms, don't you agree? And that's typical in my experience.
Depends on the type of software. I feel like PC games are on the extreme end of having lots of minor bugs and one very dominant platform.