This was a deliberate thing done by Ford for no logical reason that I can think of, all digit segments are paralleled together (all too segments are connected to one wire, all right top segments are connected to one wire ect...)
So Ford deliberately got the LCD made with the last upper left segment unhooked so the odometer could never read over 400k. Just to note also it wouldn't of cost them any money to have the segment and the program in the PSOM changed to read up to 999k.
Also for us Canadian guys this roll over happens 60% faster being in km/h than mp/h.