It's very hard to compare how your battery should be because everyones use is pretty much unique. Batteries naturally deteriorate over time. Also, according to Apple your iPhone battery capacity will be reduced to only hold about 80% capability after 400 complete charging cycles. Just charging it once a day doesn't mean it went thru one charge cycle. Its how many many discharges equals one full discharge. For example:
-- Person A - depletes their iPhone 25% each day and recharges it each day. It would take them 4 days to equal 1 charge cycle.
-- Person B - depletes their iPhone 50% each day and recharges it each day. It would take them 2 days to equal 1 charge cycle.
-- Person C - depletes their iPhone 100% each day and recharges it each day. It would take them 1 day to equal 1 charge cycle.
This graph kinda shows how different depletion/charges can total one complete cycle.
Also as stated if you don't use your device often to get a full charge cycle each month or if your battery level monitor doesn't seems to be reading correctly you can try running the battery completely down once a month and charge it back up.