You're probably seeing 14+ when full, correct? If so, that's normal. You can only measure what you see, and if, during calibration, when the float tops out at say 14, that's the most we'll ever display. So it'll say 14+ until the float comes off the peg.
If we instead treated on the peg as "full" and showed you 18, the consequence would be that when you have say 15 on each side, and you choose your stopping point based on thinking you have 18 on each side, thinking you have another 6 in the tank for reserves, well, you've just planned to arrive on empty. Now 99% of the time you'd probably catch that when your fuel drops from 18 to 14 abruptly an hour or two into your flight, but since fuel starvation is one of the most common ways to end up on the ground before you intended, we made this feature intentionally conservative.