As the top post explains (I think), if a receiver offers settings only going up to 0dB, as most/all do, then 0dB means +10 on LFE, -10dB means no gain on LFE, -20dB means -10 on LFE, -infinity means LFE muted.
+10 on LFE is the "correct" setting, so that's shown as an adjustment of 0dB. Any change lowers it from the correct setting. THX mandates that labelling. There used to be confusion, in that some receivers labelled the ranges for DD and DTS differently(!)