There are only a few boot manufacturers in the world who will make a boot to the precise dimensions of your foot and ankle. Unless you buy one of these boots, you have to start with the bootmaker's idea of how the calf, ankle and foot are shaped. To top it all off, boots are intended for a world market and feet are different in other parts of the globe. The good news is that this allows for a lot of choices. The bad news is that this makes for a lot of choices.
Some brands are good for narrow feet while others are more accommodating to wide feet.
The same applies to the ankle, heel and calf. There is also variation in fit in different lines among the same brand. Race boots tend to be pretty unforgiving and boots at the lower end tend to have much thicker and cushier linings. We also have to contend with different sizing systems and markings on the boot. Sometimes these are close to being fiction.