It's usually if you don't have a fight signed for more than a year. Mosley had the Berto fight signed before the one year went by and then immediately signed for Floyd once that bout was cancelled.
if not mistaking mosley had 2 fights get cancelled nether his fault is why he was inactive. and could be no one behind him on the list did anything in that time to move ahead.
You can't argue facts. Mosley was p4p number 3. They kept him in the rankings because he had a fight scheduled within a year of his last fight. Mayweather has beat 4 guys who were in Ring Magazine's top 10 p4p list when Mayweather fought them. Guerrero is currently ranked number 10. There's a good chance though that he'll be nudged out for Rigondeaux.