Eh, it was probably a given from the start that it was counting something.

Then immediately after the French Ministry of Magic authorizes/legalizes a new "Give yourself multiple orgasms" charm, or even just a new magical Viagra, the device's count jumps up. At that point Dumbledore knows it's counting something that correlates strongly with sexual satisfaction, but the count is a bit too low to be counting the orgasms of the entire French female magical population. Turns out the inventor was a jealous French wizard who tried to keep tabs on his left-handed wife, but keyed in the criteria too weakly, so that any left-handed witch within the borders of France would do.