Default Question about power banks

I just got a ravpower 14000 mAh. I don't know much about these things but it has a 1 amp port and 2 amp port. I also have a charger doctor so I can see how fast something is charging. My nexus charges faster on the 1 amp port than on the 2 amp. I found this odd so I searched online and I found out in order to fix this issue I would need a charging only cable. It also does the same with my note 2. The thing that baffles me is when I attempt to charge my galaxy tab, it won't charge at all on either port, it just shows a red X over the battery. Why is this? Does anyone have any tips to make this thing work like it's suppose to?

Sent from my Nexus 5 using XDA Premium 4 mobile app