I've learned to be very skeptical of the accuracy of dishing devices built into truing stands. I never assume that they are accurate.

When building wheels I generally flip the wheel over periodically and true one side at a time. That's also how I check for true. I have a truing gauge that I use for 26" and 700c wheels but mine won't work on 20" wheels so I determine dish just by periodic flipping of the wheel on the truing stand.