Why is the voltage in this digitizer reduced
The statement is that the digitizer has 100MHz bandwidth (notice the upper case M).

This implies that at 100MHz the amplitude response is 3dB down from the low frequency response at that frequency. It should really have been more explicit but that is common usage of "Bandwidth".  You can interpret the system as a low pass filter followed by a perfect sampler.

If the response is 3dB down the amplitude response is 0.7 of the low frequency value; i.e. ~0.7V