iPad USB charging fm MacBook works in left port but shows "Not Charging" in right port. Why?

I've seen the recommendations that some iPad devices need more power than some MacBooks can deliver, and it's best to recharge the iPad directly to AC.

But what explains the iPad having no problem charging in the laptop's left USB port but gets a "Not Charging" msg when plugged into the right USB port ? Both ports should operate equally, no?

(MacBook Pro 15" - mid 2012)

(see system report - "Extra Operating Current")