Cost cuts in quantity. As a guy that has dealt with machine shops and ordering custom parts, i know how cost breakdowns work...
If Vault ordered 100 or 500 it may be on Kel Tec's "low volume" cost setup. It may take 1000 or more to get good cost breakdown, and that dealers in the US buy large quantities.
especially when a company is fairly small and growing rapidly like Kel Tec. They may get orders for a certain number and have to wait to get the machines free to make parts (switch out injection molds, set up tooling for receiver/barrel machining). A company that's been around for a long time and has most or all of their initial startup costs covered can probably deal with MUCH smaller price decreases for dealers.
Say it takes me 50 dollars to make a said accessory for getting 20, but it goes down to 30 bucks if I order 50 or more... If I own the machine to make those parts, it costs what I pay for the material plus the electricity (if the machine is payed off) so if I ship only 10 to a customer, I can get away with selling them for a low price point of 30 bucks...