Need advice regarding a ups

Hello everyone i have a ups SRV15KUXI-IN(11)

And 2 battery pack

SRV120RLBP2-9A

When i adjusted my battery AH to 18

It gave a fake runtime 22 hours and cant hold more than 2:45 hours

When i revert it back to 9AH it show 3 hours runtime which is more logic so what do i miss ?