There's a fair amount of reference material on the web that states that the maximum label length for an exFAT volume is supposed to be 11 characters. However there are two ways that I am aware of to exceed this:
Use the Linux tool
exfatlabelfound in the exfat-utils package;
Format a USB device using the "Format" option from the "Right click on the drive" context menu under Windows 7. Note that the same option under Windows 8.1 no longer allows labels longer than 11 characters.
Both of the above allow a volume label up to 15 characters long with no other apparent ill effects.
Any ideas as to what is going on here? The fact that Microsoft corrected this issue with the transition from Windows 7 to Windows 8.1 suggests it's probably a bug, or some other sort of undefined or otherwise questionable behavior.