I think that in general, fans think that more riders should have won "multiple GT's" then what is realistic. Consider that most riders only have about a 5 year peak period, and that there are only 3 GT's each year. To make matters worse (or harder), most riders can only peak for one GT per season (it's not like tennis grand slams), and there is probably going to be one rider during your era who is quite dominant, and who wins at least a few GT's. So if you are a rider who was probably in the top 10 of the GC riders in your era, but probably not in the top 3, and someone who doesn't stand out from half a dozen of their other rivals; then how can they be expected to win multiple GT's?
Someone like Menchov (who won 3 GT's) probably greatly overachieved, rather than his rivals (Kloden, Evans, Vino, Levi, etc) greatly underachieving, if that makes any sense.