This is the text from the mentioned book affiliated to the posted figure 11-21:
Any voltage can be applied to the front end of a
differential pair, such as a microstrip differential pair.
If we were to launch the voltage pattern of a 0-v to 1-v
signal in line 1 and a 0-v constant signal in line 2, we
would find that as we moved down the lines with the
signal, the actual signal on the lines would change.
There would be far-end cross talk between line 1 and
line 2. Noise would be generated on line 2, and as the
noise built, the signal on line 1 would decrease.
Figure 11-21 shows the evolution of the voltages on the
two lines as the signals propagate. The voltage pattern
launched into the differential pair changes as it
propagates down the line. In general, any arbitrary
voltage pattern we launch into a pair of transmission
lines will change as it propagates down the line.
Is no information about the length and terminations of the differential pair lines (should be).
Later in the book the author speak about cases when the differential pair lines are terminated, which confirm that the lines in figure 11-21 are not terminated.