The FCC table of frequency allocations clearly states on page 27 that the Amateur allocation for 70cm is 420-450MHz, but I've seen a lot of transceivers that limit transmit to 430-450MHz.
Why is this?
It is probably due to the fact that Canada does not include 420-430MHz in their 70cm band plan. The United States agrees not to allow their amateurs to interfere, by restricting 420-430MHz north of Line A and east of Line C. (See also FCC Title 47 Part 97.303.m which I can't find a good way to link to.)
When the manufacturer decides to restrict 420-430MHz, then the transceiver can be sold in both the US and Canada, and US customers are unlikely to mistakenly infringe.