Confusion over PvP Achievements

I'm a bit of an achievement hoarder. I've been thinking of getting started with PvP ones, but I noticed something odd. I do wish to note that I had been switching between grand companies to get achievements for all 3:
Maelstrom Victories: 2/30
Twin Adder Victories: 13/30
Immortal Flames Victories: 1/30

Now onto the general victories:
Front and Center III: 13/30
Behind Enemy Lines I: 13/200

Why is 13 the number listed? "Behind the Enemy Lines" says "Guide any of the three Grand Companies", so maybe I could see that it only applied to 1 Grand Company, the one with the most victories, not all 3 summed up. However "Front and Center" is a completely general "Triumph in X matches in Frontline".

Where does this difference come from?

It is like you said. Since always. In the past choosing gc wasn't freelancer, you had to stick to your gc and couldn't switch to the winning ones all the time, that might be a reason why only one of three counts.

Devs just being geniuses on the description. The PvP achievement titles and reward structure don't make sense at a lot of points. Japanese titles are often better (some are even in english but they re translated them to something totally different in english), and getting a node in seal rock only ever contributes if you are the person doing it till 100%, things like that kinda suck.