Odd VSI behavior at higher altitudes

I have a PA28-180. Whenever the winds allow, I like to fly at high
altitudes. On my last few flights, I have noticed that the VSI tends
to show a slight descent when I am at high altitude. The altimeter
will stay constant/steady (as does my Mode C readout). The VSI does
not exhibit this behavior at lower altitudes.

So for example: At 3-5kft, the VSI seems to be dead on. At 10k and
above, the VSI shows a 30-50fpm descent to maintain level flight.

Thoughts?

Thanks,
Nathan