To put it simply and I'll say the same with LIGHT even. It doesn't dictate skill; it dictates the amount of time you've spent on this game. I constantly have friends saying they're better than me because they have a higher light. I don't raid very often and my closer friends raid weekly so of course your light will be higher. GRIMOIRE is the same way. Gather a few ghost collect a few exotics and play a few games of crucible and you could notice a significant increase in grimoire.