Yes, you are right. These numbers must be used carefully. There is a lot of overlap between PvE and PvP, as players who do both on a single day will be added to both numbers. As for gambit, before it was added to Destiny, the PvE/PvP ratio always hovered around 3/2. With Gambit, it stabilized around 5/4. I don't know it it was caused by more PvE players playing Gambit, or by what you mention (PvP players being counted twice if they play Crucible and Gambit on the same day). The total number of daily players can never be known, only the low and high bounds. I'd like a better metric, for sure.