Check your right side function panel. The one that looks like this
Your jump range has two numbers. The first number is your
CURRENT jump range. This number is always changing depending on cargo and the fuel you have on board.
The second number in parentheses is your maximum possible jump range,
if you have no cargo and only enough fuel for one jump.
You cannot exceed your
current jump range. The maximum range only applies if you can get your current jump range to match the max by removing cargo and fuel.
I'd guess you probably took on some cargo and your range is now reduced.
Some missions give cargo as part of the mission reward so you may have cargo on board you didn't know about.
If I'm completely wrong and your current jump range is still 15.2ly, then it's probably a rounding error.
The game tends to round off numbers at varying decimal places, so the distance you're seeing to the destination could be more than 15ly but the display is being rounded down to 15.