Let's think logically for this. Before going into the pump, what is your normal erected length? I'm assuming 14cm?
When you're in the pump, are you erected or flaccid?
When you're flaccid, the penis tends to be much more "stretchy" than when you're erected. The reason why is the restrain of tissues by the tunica and septum bands. They have a limited amount of "stretchiness". When you're erected in the pump, that stretchiness is strained, limiting the extra length you have in side the pump.
On top of that, if you have a fat pad, the pump pushes against the fat pad at the base of the penis, exposing that extra cm right out the gate. If you have never pull on the ligaments and base muscles before for lengthening work, in additional to the fat pad, you also have a hidden segment that's being pulled and hold by the suspensory ligament and the base muscles. This is why a large amount of cm length suddenly comes out in the pump that you have never noticed before.