a) the matrices of width vs height vs style of jeans available are filled in on a store by store basis
b) they are a tissue of lies
in that the chart in the Oxford store, unlike the Reading one, didn't have 33 inch waist, 36 inch inside leg ticked. However it turned out that they had a pair of 32 waist, 36 inside leg (not ticked) blue jeans and, even better and much to my astonishment since I have only found a similar pair once before in my life, a pair of 31 inch waist, 36 inch inside leg (not even on the chart) black jeans. So what can one say but, to use the vernacular of hereabouts, w00t! Both for their existence and the fact that they fit!
I have also bought 'Jonathan Strange & Mr. Norrell ' to read on my long tedious flight, though it doesn't sound 100% my sort of thing. I must admit that I did briefly have a look at 'The System Of The World' before coming down on the side of JS & MN - I don't know quite what would have induced me to buy the Stephenson but the random passages I read certainly didn't.