You have hit on what we consider to be the core value of the tool. Please enter your 27 cm by using the slider on the sizer and then hit the "internet shoe size" tab. You will find that the USA Shoe size value changes to the size 10 that you had mentioned. Yet if you look inside your size 10.5 snowboard boots they will always have mondo 285 printed inside. This indicates by the definition of the mondopoint standard that the boot manufacturer is suggesting that boot for a foot length of 285 mm.
You mentioned Zappos and for good reason. They (Amazon) are the 820 lb gorilla in the shoe room. Their shoe chart does not relate to any standard. It is also used by other online shoe sellers and it is an entirely recent creation. We use those values on the "internet shoe size" tab on the sizer because they have become so vexing to the footwear business.
Here is the underlying fact. When the average person buys shoes online and has them shipped to them they are statistically much
less likely to return a shoe for being too large than too small. Too small always goes back. This carries a significant expense to the retailer, especially ones that offer free return shipping. As that reality has become apparent we have seen the "correct sizes" on these charts go up and up.
The result? The average shoe size sold has skyrocketed in the last decade. Has the average foot size changed that much? Shoe and boot fitters will relate that it is now most common to have an average dimension size 8 foot pull out of an 11 shoe and so on.
It is this huge discrepancy that interests us. It is also important for new riders that are entering the buying process as they will find volumes of online advice suggesting that "shoe size" (which shoe size is never noted) or mondo size be used as the starting point for selection, but these values can be separated by 2 shoe sizes
. This leads to a ridiculous and correctable
amount of terribly fit boots being sold both in shop and online. This does concern us as poorly fit boots more than any other piece of gear will turn off a new rider and send them back to Playstation.
This is in no way to tell you that your boots don't fit.
It sounds like you have a wide foot. Is that the case?