We carry one brand of footwear that builds their 'last' or foot-shape pattern
for each style/size to 'Brannock' (the sizing tool we use), or true measure.

If you measure to a size 10, your toes will probably touch the end of a size 10 shoe.

Another builds to 'proper' (my term) fit, meaning that their size 10 leaves room at the front.

In the first brand, you'll be more comfortable in a 10.5, in the second, a 10.
Once you know their design/intent, the variations make a bit more sense.
Since both are consistent in this, it makes things easier.
That's all good and well, but such differing systems are invisible/unadvertised to the consumer. We never know, when we walk into a store and see 20 different brands of shoes, which size is going to be correct for us.

It's not a huge deal when you can try them on and ensure a good fit. But it makes it incredibly frustrating when buying online. It's also frustrating for those of us with a shoe size right at the edge of their production line. I wear a size 7, depending on the shoe. In some brands, that fits me. In other brands, it's too big. And they never offer a size smaller than that. Sometimes they don't even make shoes smaller than size 8.

(Sorry for the whiny threadjack.)