[Samtools-help] samtools mpileup seems to be ignoring -Q base quality filter?

Hi,
   We are finding that mpileup is reporting bases that are well below the 
minimum requested base quality set by the -Q option. The following shows 
an example:

samtools mpileup  -B -f ucsc_all_ref.fasta -r chr3:49932645-49932645 -q 20 
-Q 20 test.bam
[mpileup] 1 samples in 1 input files
<mpileup> Set max per-file depth to 8000
chr3    49932645        G       74 
.$,....C........,,.,,.,.,,,,.,..,,......,C...,c,,.....,,,,,,.,.,,,.,..,.,,, 
 
DE4FDG(IIGGJFFIB%=IHFHH%HHIH%JJE%GGFJHG%,9GHG67HHHIIHAHFHH%I%H%%GHIIIHHAF7
The quality string shows that the base quality of the first C is '(', 
(numeric 7) and that of the second is ',' (numeric 11). The third has base 
quality 21 and so it should show up here, but the other two should not. It 
doesn't seem to matter what value we use for -Q, nothing ever seems to be 
filtered out. Use of -B does not affect this behavior.

This sounds similar to the problem reported below as well.
http://biostar.stackexchange.com/questions/17030/filtering-out-reads-based-on-base-quality-with-samtools-seems-not-to-work-properl

Thanks,

John