You don't give enough info to provide much of an answer.
Which 18650
batteries are you using?
Are they authentic cells? Bought from a known reputable vendor/dealer.
Are the cells identical? ~ same brand, mah, cdr, age, cycles...
What are you using to gauge the different discharge rate? ~ display screen on power device, external/stand alone battery charger, multimeter.
It isn't uncommon for multi cell power devices to charge or discharge cells unevenly noted by battery level indicator on display. Charge cells using a quality external charger and rotate battery positions in the power device / mod battery sled/slots.