I've also had significant variance powering the same device on the same model plane. As others have stated, it could have something to do with how drawn down the device is when you attempt to charge.

Bottom line is if you want power, you need a portable charger and/or backup battery. I'd also suggest bringing your own portable entertainment device with content loaded. (I bring my own portable chargers plus my own iPod video loaded with content).

I learned the lesson loooooong ago to never rely on ANY airline to keep me entertained on ANY flight. I still can't believe how many risk 10+ hours of boredom when AVOD goes down then they come on here and piss and moan about it on these forums.