You are consistently beating $100NL, it's not something you should fret about.

I wonder if this is what's happening: As you get more comfortable with your game and your surroundings, you are subconsciously (or consciously?) exploring new hands, new moves, new ideas, which is allowing you to open up and play more hands in more positions. You are probably playing a few hands which are marginally -EV but it's okay because it is helping you grow as a player, find your limits etc. As a result, you're losing a few BB/100, but because you're not making any big or terrible errors, you don't really notice where the returns are dropping.

Tbh it all sounds quite encoraging - you're continuing with your poker education through trial, error and experience, which is really the only way it can satisfactorily be done. Take an evening off poker to study Poker Tracker - have a look at the hands you're playing now; check your position stats (a bad habit of mine is playing SCs too early, for example - this sounds like something you might be doing, hence you'reincreased VP), and look at some sample hands, check if you're happy with how you played them and what you'd next time in the same circumstances.

It's great that the best of the best have double-figure BB/100 figures, but the vast majority of us don't. You're a consitent winner and obviously a thoughtful player who is still keen to learn. Don't introduce unnecessary panic and you'll be fine.