sort-mailbox by spam tag score sorting strangeness

[bob@xxxxxxxxxx (Bob Proulx)]

I have the following set:

  spam "X-Spam-Status: (Yes|No), score=(-?[[:digit:]]+\.[[:digit:]]+)" "%2"
  set sort=spam
  set index_format="%4C %Z %{%b%d} %4H %-15.15F %4c %s"

This is to parse spamassassin headers such as this:

  X-Spam-Status: No, score=-0.2 required=5.0 tests=BAYES_40,UNPARSEABLE_RELAY
          autolearn=ham version=3.1.3

This seems to get confused about the sign of the spam score.
Sometimes it seems to get it right.  And sometimes it does not.
Here is an example, with the names and subjects blacked out to protect
the innocent.

  69     Nov13  0.1 XXXXXXXX    2.6K XXXXX
  70     Nov10  0.2 XXXXXXXX    2.5K XXXXX
  71     Nov10 -0.2 XXXXXXXX    2.5K XXXXX
  72     Nov10 -0.2 XXXXXXXX    0.4K XXXXX
  73     Nov11 -0.2 XXXXXXXX    0.7K XXXXX
  74     Nov13 -0.2 XXXXXXXX    7.0K XXXXX
  75     Nov13 -0.2 XXXXXXXX    6.7K XXXXX
  76     Nov10  0.5 XXXXXXXX    3.7K XXXXX
  77     Nov10  0.5 XXXXXXXX    4.3K XXXXX
  78     Nov14  0.5 XXXXXXXX    3.4K XXXXX
  79     Nov11 -0.6 XXXXXXXX    1.1K XXXXX
  80     Nov11 -0.6 XXXXXXXX    1.0K XXXXX
  81     Nov10 -0.7 XXXXXXXX    0.8K XXXXX
  82     Nov10 -0.7 XXXXXXXX    1.3K XXXXX
  83     Nov13 -0.7 XXXXXXXX    2.1K XXXXX
  84     Nov14 -0.7 XXXXXXXX    0.9K XXXXX
  85     Nov09 -0.8 XXXXXXXX    1.0K XXXXX
  86     Nov11  0.8 XXXXXXXX    0.3K XXXXX
  87     Nov11  0.9 XXXXXXXX    0.9K XXXXX
  88     Nov11  1.0 XXXXXXXX    2.9K XXXXX
  89     Nov09  1.1 XXXXXXXX     90K XXXXX

Negative values less than -1 all seem to sort okay.  It seems to be
only the absolute values <1 that cause issues.  Why is it sorting this
so strangely?  Why does it appear to be taking the absolute value of
the score but only if the score is between -1..1 but is okay outside
that range?  This is mutt 1.5.9-2sarge2 from Debian Sarge stable.

Bob