Voltage between ground and positive / negative terminals, is this correct?
The outputs are almost floating with respect to ground but a tiny feedback current both +/- will show the voltage with respect to ground as an offset when only loaded with a 10MOhm probe.  

My guess is that is normal for your old model and wont have any impact in most applications.    I suspect newer models improved that leakage. But this wont stop you from using tracking mode or Independant or CV , CC or same or complementary polarity earthed at any one point.

Typical use in the old days was separate Analog and Digital grounds wires but the complementary 0~24V outputs grounded at the PSU only with the 5V logic ground, so they do not share logic current noise.