My el-cheapo but comfortable sneakers wore out, but the store had some that were almost identical. I tried on a pair that were labelled as being the same size as my existing pair but they didn't fit. The next size up fitted nicely. Yet on closer examination, the shoes that fitted were identical in actual size to the earlier pair, even though labelled differently.
Yes, they were cheap, and, yes, both pairs were made in China - but it does serve to show just how unreliable the sizing system is. How anyone manages to buy items such as these over the internet without the ability to try them on is a mystery to me.