The BB fighter has average Dexterity and Perception, both 10. When he suffers a reduction of any of this two, the resulting value of the Reflex defence is by 1 point higher than it should be. I did some tests with different builds and spells -- and it looks like this issue holds for all the defences modified by the low sum of connected attributes.
The picture below shows different characters suffering by the bug:
For example, the first character has:
Deflection 25, Fortitude 19, Reflex 19, Will 19
-> Should have:
Deflection 24, Fortitude 18, Reflex 18, Will 18
and so on with others...
How to reproduce:
A) either, by putting Hobbled or similar effect on BB fighter -- the reflex defence will be off;
B) or create a new character from any of those in the picture above or similar (sum of two attributes modifying the tested defence to be < 20);
According to several tests with different setups, the bug seems to occur only when the summed modifier from attributes is negative regardless to its value.
It's like the game does something like this for Deflection:
modifier = (Per - 10) + (Res - 10)
if (modifier < 0) --> deflection = deflection + modifier +1
else --> deflection = deflection + modifier
And for Reflex defence (and similarly for For. and Will):
modifier = (Per - 10)*2 + (Dex - 10)*2
if (modifier < 0) --> reflex = reflex + modifier +1
else --> reflex = reflex + modifier
Did some basic tests with attacking the corresponding defence and it seems the numbers are the same (wrong) in the combat log.
Leveling up also doesn't help with the issue.
Note: The first two character sheets also show the bug with bigger fonts (130%). The longer Background descriptions don't fit the sheet size.