Well, in my limited search I could not come up with a direct answer so here is my best guess.
Your manual is right. Your 2013 has the 5spd tranny and is EPA rated at 16/23 mpg. The 2014 has the 8spd and is rated at 17/24 mpg.
Different tranny, different tune = different octane requirement.
I makes sense they would de-tune the motor for 2014 with the 8spd and stiffening CAFE fuel standards.
Use 91, any more is 100% wasted money. IMHO you could use 89 since the ECM will sense the lower octane and auto-adjust timing to make things run hunky dory. FYI, my wifes' Pathfinder V6 called for 91 and we ran 87 octane for 197k miles and it still ran like a champ when I sold it last year. Motor purred. just sayin'
Oh, btw, if you tow, then run the 91 for the tow...it needs it under load....