Default [Q] Portable battery charging speed

I have a TENERGY 10000mAh battery charger with 2 ports. A 2.1A and a 1A.

Any reason why the 1A charges faster than the 2.1A? I have the battery monitor app.

Thanks

Sent from my Nexus 5 using xda app-developers app
Galaxy nexus GSM Yakjuzs