No actually I had to go to work, but once again this is all speculation, with comments like "seems" in there does not make the statement anywhere near conclusive. The only people that can confirm or deny this is the development team. There is math that does determine the boundaries of actual damage reduced, but ultimately that is just a set threshold, everything else goes off of the tic function which is predominant in almost any programming language that generates a "random" but not really random number within that threshold, therefore it is entirely possible that guard skill at higher levels can reduce more damage taken as that threshold can change with the values of the skill.