You should time it with a stop watch as the time in app changes depending on how your flying... like hooning around will estimate lower time remaining than just hovering.
Also... I only get around 15 minutes.. Mine is just under 3 months old, but got same results from day 1.
I get an extra minute or so now that I changed to 9450 props, but beware some, if not most people lose a minute or so with those props.
I hear what you are saying though...and I to am curious why we get what seems to be less time than others. It also obviously depends on flying styles. If i do JUST hover I will get closer to 20 minutes.. JUst recently I have started reading more thoroughly into proper battery care to try an increase the life.. There is some documentaton on the dji website under downloads section that you might want to read through if you have not already (stuff like run down battery as you mentioned).
but yeah.. seems like some of us definitely do loose out on a few minutes... I made a thread similar this... someone said its only 3 minutes.. and I was like yeah but I have done 100 recharges across both batteries (I get the same 15 mins out of both batteries) so its like 100x3 = 300 minutes!!
I also took it back to the store I got it from after having it for 2 weeks after I noticed I could only get around 15 minutes, they couldn't find anything wrong with the phantom, 2xbatteries.. all they did was give me a new charger, which made no sense and did nothing... I got over the 'problem' for a couple of months, but yeah as I said I'm back into researching this.. would like an extra 2-3mins for free
Was having some major major battery issues recently (not related to this) I think I have finally resolved so I can go back to enjoying flying.
BTW How old is your Phantom ??