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Hi Ron,
This is not what is causing the 1/2 gain in one of the two channels but not the other. The 7A26 is not differential. It has two separate input channels.

In certain cases the 2nd channel can be inverted and added to the first channel to make a "primitive" difference, but not differential, amplifier. Tek generally makes no claims about the performance of their vertical amplifiers when the signal goes off screen. While the signal is on-screen it is in the linear region of the vertical amplifier and you can expect the bandwidth of the amplifier will meet its stated specs.

Provided neither signal by itself goes off screen, and the addition of both signals stays on screen, you will probably remain in the linear region of both channels. If you invert Ch 2 and add both signals together you will get the difference between them provided you use the same probes on each channel and you are careful to calibrate their response as closely as possible. You will also need to calibrate the gain of each channel.

Even if you did all this it would be misleading to assume that the on screen result was what a true differential amplifier with matched probes, matched inputs, matched phase response, matched offsets, etc would give you.

Dennis Tillman W7PF