L-Nav Speed to fly Indications

I have an L-Nav connected to a Model 20 GPS and also a Winter
variometer. While at Marfa I had several oppotunities to fly some
long distances without having to thermal. What I noticed is that the
speed to fly commands are really off. The other variometers seem to
track each other and the averager and netto displays on the LNav seem
to jive. But, when flying straight, and flying through heavy sink,
the push to pull commands are showing me to slow down, just the
opposite of what is expected. The altitude needed to get home seems
to be correct, it was almost the same number as shown on the PDA with
GNII running.

Any ideas,
Fred