Thanks for taking the time to demonstrate.
For us with periods for decimal separators, when you say “x,5” you’re saying “x.5”. As in, an 8+2 range Donjon can actually fire at an archer 10.5 tiles away. Thus, the archer’s health is going down in the image you show, where the archer is within the 10.5 radius gray range indicator circle but more than 10 tiles away shown the by the gray grid indicator.