If you mean the barrel on the upper is 10.5 inches, it's because to avoid making it a SBR with a stock on the lower and having to go through the BATF for a Class III/NFA Tax Stamp for it, you'll need to go with the AR pistol config option for the lower which means you'll need a pistol buffer tube assembly for the lower that you attach to the 10.5 inch upper. If you do not have the Short Barreled Rifle approved through the BATF and you put a 10.5 inch barrel length upper together with a lower that has a stock on it, you've committed a felony in the eyes of the BATF. That's why most folks go with the pistol config of a short barreled upper with a pistol brace on the lower. Please see image link below for my 7.5 inch .300 Blackout AR pistol with an arm brace.
https://i.imgur.com/BNCO54Y.jpg

Hey Daryl, any upper with a shorter than 16 inch overall length with anything other than a pistol buffer tube is considered a SBR (short barreled rifle). With a standard stock (including a "Bump Stock") this becomes a NFA Class III item and you would need to register it and with the appropriate paperwork and fees. You certainly don't want to inadvertently build something that could get you a reservation at "Club Fed".