Do you mean the low bitrate? That is indeed pretty low, but not unheard of - TrueHD does a pretty good job of compressing out silence, so if the surround channels are frequently inactive then it's probably reasonable for a 16-bit track.
Here is my list of 5.1 TrueHD tracks ordered by bitrate ascending (apologies if this page is slow to load):
I haven't done it myself in awhile, but you can spot check numbers by demuxing the raw streams using eac3to, TSMuxer, etc. and dividing stream sizes by duration. It's kind of pain in the ass, although this is how benes et al. came up with the original specs prior to BDInfo. Worth noting that if you demux a single stream into a container like M2TS instead of as raw stream, the numbers will be ~10-15% higher than what BDInfo reports.