My pvc back door has a raised doorstep that I have to step over to go outside.... it's part of the frame, so I suspect that would account for the difference in height. Anyway, yes the doors are made to measure (well, mine was !)
NB I would not go anywhere near that particular company - their product may be good but they are exceedingly expensive (over �2000 for each window is what they quoted me !!!!!!) and trying to get rid of the saleswoman - she stayed for 4 hours - was hell on earth.... I had to eat my dinner whilst she stayed there, trying to persuade me to take out a loan with her company as well (to pay for the whole house to be fitted with new doors and windows).
Go to a local company if you can, preferably one recommended by a friend or neighbour. Look around at who has had windows/doors fitted recently and ask how the work went.