Seem's like a problem with inconsistent values, best to give example.

From this entry (1.c4 c6 2.Nf3 d5 3.e3), game explorer says that 19 games had the move 3 ... Bg4.

But after clicking into (3 ... Bg4), game explorer says that there were 171 games that had that position (171 = 95+58+9+7+2).

Am I misinterpreting the information, or is something incorrect with Game Explorer?

nothing is wrong with explorer IMO, you just transposed into position which can be reached via various move order. Try 1.e4 Nc6 2. Bb5 e5 , and you wil find a few games only. Then hit Nf3 and suddenly you will jump into Ruy Lopez with so many played games.

thanks, that makes sense.