I've collected size charts for every shoe model across 200 brands and found that it's not uncommon for any given size to vary +-1.5 sizes (a range of 3 sizes total). And that's size charts from manufacturers.

Many online stores use outdated or outright wrong sizes charts. One of Europe's largest online retailers don't even bother providing official manufacturer size charts at all. They have one generic size chart for almost all of the shoes, so it's pretty much down to the consumer to return and retry. Kind of crazy state of things.