Aww... Don't fret it. I understand what you mean. (IIRC, 500 is the cutoff since 10 pages are all that is kept, 50 per page.) But welcome back. How are you?
Likely you had two PM notifications that were not checked as unread. 10 pages, 50 each, /should/ be 500. But, maybe there was a glitch or something that it cut off at 498 for you.