Why oh why does the cycling industry seem to follow a totally
different and random sizing scheme to everyone else? I'm size 45 for
any other shoe, but I need size 47 shimano shoes to fit, and they are
firmly snug in both length and width.

It's a sports thing. I've got size 11 feet, but all my football boots are
size 12. Shimano shoes fit me perfectly if I go up to size 12, as do Puma
football boots. Most get too wide when I go a size up.