SAT in low bass

[Ted Simmons]

Hi list,
   While we're on the thread of using SAT's for piano tuning, I'd like to
relate an incident that happened recently.  I was called in to tune a
spinet that I had never seen before.  I set up my SAT for this piano and
began the tuning.  As I got into the lower bass the SAT indicated that the
notes were very flat and had to be adjusted upwards.  When I got to the
lowest B, however, the  SAT indicated that it was only slightly flat and so
I merely tweaked it a little.  I was puzzled by the fact that among all of
those "very flat" notes was one that was almost on pitch.  So I did some
aural checks and, while striking the lowest B, I played notes up from it
one at a time and discovered that this B was in unison with D1.  Instead of
being flat, it was sharp by a great amount!  I love my SAT and it performs
well for me, but it fooled me down in the low bass.  I've been leery of its
bass capabilities ever since.  Can anyone explain why this happened and how
the SAT can be used to home in on the correct pitch for these low notes?

Ted Simmons, Merritt Island, FL (1 hour's drive from Orlando)