Unsurprising, given Quickload's limitations.

It is probably due to the burn rate of the batch of powder you have differing from that used to create the powder data file, the inability of QL to model freebore or any of the many other factors QL doesn't model.

This is why despite having the program, using it and liking it very much for some applications ( mainly those "what if" type scenarios that would be very expensive to try blind) a chronograph, at about the same cost as QL, is a much better investment for more reloaders.

Having said that, velocity is a function of pressure and time and so when you top out on velocity compared to reputable lab data you have probably topped out on pressure too. This can be masked by the excellent Lapua small primer brass which handles pressure far in excess of SAAMI or CIP specs very well.

What where you trying to find out, out of interest?