BODLEVEL Fuse Coding
It means that the exact BOD voltage is a little imprecise and can vary within those limits. Take the strictest limit into consideration considering your implementation.
For example, if MCU is directly on battery and with BOD=1.8V, it will go into reset at some voltage between 1.7V and 2.0V (same voltage every time for the same MCU, but different for different MCUs, all between 1.7V and 2.0V), and you should consider 2.0V your real BOD voltage as you don't want to have half devices function at V=1.9V and half not. You aim for guaranteed predictable behavior.