There's a setting for when to warn you of a low charge and another one for when to turn the phone off due to a low charge. If you have the turnoff set to 15% and the warning set to 10%, you'll never get the warning, the phone will turn off at 15%
Another possibility is a dendrite forming in the battery when it gets down to 15%. That shouldn't matter, because you should
NEVER allow a lithium battery to drop below 40% (unless you like getting a new battery every few months). See
The Care and Feeding of Lithium Polymer Batteries.