Common voltage when use a battery powered DMM to measure a voltage
For a common-mode voltage to be defined you need 3 wires !
For an explanation about common-mode signals, go here and scroll down to figure 3:



Not how there are signals A and B but also a common (ground, 0 Volt) line.

Now look at your handheld DMM in Voltage mode, it will only have 2 connections.  If you would connect these 2 between signal A and signal B you would measure 3 V (4 V - 1 V).

Now look at your signal generator, it also has 2 connections but one is grounded so you are only to either make signal A or signal B.

A handheld DMM cannot measure the 2.5 V common-mode voltage directly ! It would need to have access to all 3 lines: signal A, signal B and common.

But you can measure the common-mode voltage indirectly. You first measure signal A with respect to ground, this is 4 V. Then you measure signal B which is 1 V. Then you calculate their average:

(4 V + 1 V)/2 = 2.5 V

which is the common mode voltage.