One don't use the bark
Your answer is size in the different reading. I you have just the 2 rod meter your only getting a surface reading (RELITIVE HUMMITY). If you don't believe me. Take a drill bit 1/6 Th drill 2, 1/2 inch deep somewhere it wouldn't hurt your bow.
Check the surface at the holes. Then put you pins in the holes. A 15 year old stave's won't read the same.
Now that was 15 years, now unless moisture meters have really Improver. Just used the old system. Just give it another years. Most white wood stave's stored it a dry ventilated place (that aren't tree's. Have seasoned good enough in 2 years to make a bow. Of course 5 or 6 years is better.
You have a lot better chance making a nicer bow from a stave that a board.