The Mac EFI BootROM boot volume picker may be reading the GPT labels for the partitions without looking inside the partitions and parsing the HFS+ volume format to find the name the volume calls itself internally.
See if this command shows you the same names that the boot picker shows:
sudo gpt -r show -l disk1
Note, your USB drive may not be disk1 on your system. Use diskutil list to determine the drive device number of your USB drive, and use that in places of disk1 in the command above.
If the command above confirms my hunch, you should be able to change those labels like this:
sudo gpt label -i 2 -l "Install OS X Mountain Lion" disk1
Note that I assumed that the Mountain Lion partition was at "index 2" in the output of the gpt...show command above. You should replace 2 argument of the -i option with the actual index value for the correct partition, as told by the index column of the gpt...show command output from your system. And of course, replace disk1 with the proper disk device identifier for your USB drive, as I said before.
There's a chance you won't be allowed to relabel a drive that's in use (editing partition tables of drives is considered dangerous, especially when in use) so you might have to make sure none of that USB drive's volumes are mounted when you try to do this.
Standard disclaimers for partition table editing apply: Mistakes made while editing partition tables can cause you to lose access to all the data on your drive. Make sure you've backed up everything important to you (and TEST YOUR BACKUPS) before proceeding to edit a partition table. Be very careful about your disk device identifier so that you're editing the partition table of the drive you really meant to edit.
