My understanding is this...
As a spring expands it has an energy curve associated to it. Tempering of metal, coil diameter, coil spacing, wire diameter, etc...will all affect that curve.
For an AEG...IMO...springs would serve the system best if they could accelerate as fast as possible and hit it's peak power just as the piston head reaches the cylinder head. The tighter pack of coils is supposed to assist with doing that.
As a spring is compressed going into the mechbox...some quite a bit, others less...the tail end of the power curve isn't as important.
There was a really good article about it somewhere (not on this site) where someone actually graphed the power output of some springs. Can't remember where though.
That cylinder is not mismatched for that length of barrel....but he could have the cylinder in backwards too
(although the FPS should be much, much lower than what he was getting).
Also..he could be not shooting straight through the chronograph, which would give a false reading (usually higher though)...some guys seem incapable of lining things up
...he could have his muzzle too far away from the chronograph (which would give a lower reading)...one of the easiest ways to cheat a chrony is to just stick a suppressor on your gun
...he could have his hopup cranked on a lot, which shaves off tons of FPS
There's lots of reasons...and too little information to do much more than just toss quick guesses at it