not allowing me to change gamertag to specific username despite it not being taken.

So recently me and some of my friends decided to all begin our gamertags with BoLT as we thought it was reminiscent of when people used to start clans on call of duty. i chose the name "BoLT " i checked the see if the gamertag was taken and turns out it wasnt, so i made a new account and tried giving it the gamertag "BoLT " but according to xbox it was taken and it tried giving me ugly numbers next to the gamertag. (i wanted to grind levels in old call of dutys so i made a new account instead of changing my main account) i searched the gamertag and noone had it so can someone please explain why this is happening.







:)