Staff Master
There’s a weird issue with things being killed by a damage source that doesn’t include your equipped weapons. For instance, if you kill things with certain kits, it will progress things like staff master or sword master instead of pistol or rifle master. I’ve never really tried it with turrets, but it might be a similar issue.
If you just focus on killing it with your pistol and not use kits and such, it should progress the pistol track.
When you look at how many tool tips are inaccurate even after al this time the game has been out, one can only imagine how many things are flagged incorrectly on the back end that we never see. Those things are obscured to most players, so they do not get brought to the fore front enough to get fixed most likely.
One of your kits is probably flagged as a “staff” weapon.
Just like killing something in a Golem in WvW is counted towards your hammer mastery.