Maybe someone can tell me what went wrong here or how "that's just not possible...".

I always fly starting w/fully charged batteries. Last week I put in one of my genuine DJI 5200 mah batteries & took off with the Phantom 2. I always "hover" & check things out before "corncobbing" it. Battery good....probably read 95%  or so, that's the highest I ever see. So, I gave it full throttle (climb) & full forward. The drone was still in eyesight (per FARs) passing over a corn field heading into a bean field @ 400'. Only a minute or 2 had passed at the most. I glanced down at the monitor screen & couldn't believe it......"12%" remaining!!!! How could that be? Had I fallen asleep standing up? I still had control. I immediately started a descent & bringing the drone back to me. Then I thought...I might have an easier time finding it after the inevitable crash landing, in the beans vs the corn it was about to overfly. In hindsight reality, it probably wouldn't have made any difference though. But, I kept on flying back to the take off point thinking that at any second I'd be testing the drone's glide-ability (= 0.0) from 200' or so. The battery life-o-meter was not winding down as fast now. But, I had never flown with single digit battery numbers either.

Much to my delight & surprise, I landed it soundly on the grass not far from me. Battery was down to 2%.

So, how was all this possible? Shouldn't that Phantom 2 have turned around & started coming back @ 20% battery? It didn't. The "20%" must've happened within about 45 seconds after take off.

After landing, I put in another fresh battery & completed the mission. No problem. I threw away the apparently defective battery when I got home. I think you're supposed to dispose of them in some correct manner....I did. Ordered a new generic battery & all is good again.

One thing to add, I did notice when I charged that apparently defective battery, it did take abnormally long. But, the battery lights eventually quit flashing & indicated full charge.

Any thoughts as to why this happened?