Winter Vario

When circling at the normal thermalling speeds my Winter vario seems to
reflect the sailplane actual rate of climb however when flying at a
higher constant speed (example 100 knots) the vario does not reflect
the rate of climb. I first noticed this when penetrating into a wave
where the vario was giving a positive rate of climb (example 4 knots)
for several minutes however the sailplane continues to descend. Is
this normal?

I also have a 302 however I have developed the habit (probably bad
habit)of looking at the winter vario and I can't really say what the
302 was indicating although I believe it to be more accurate.