The remark about the 25 MHz BW is incorrect.
The scope has an analog BW of 100 MHz on ALL channels.
Note that analog BW and digital sample rate are two different things.
Analog BW = 100 MHz on ALL channels, even if you use all 4 channels at once.
Digital sample rate is 1 GS/s when you use 1 channel, 512 MS/s when you use 2 channels, 250 MS/s when you use 4 channels.
For a 100 MHz BW, 250 MS/s is enough to represent the original signal as it complies with the Nyquist theorem.
The Nyquist theorem states that a signal must be sampled at a rate greater than twice the highest frequency component of the signal to accurately reconstruct the waveform; otherwise, the high-frequency content will alias at a frequency inside the spectrum of interest.
More detailed information about the relation between Bandwith, Sample Rate and the Nyquist Theorem is available here:
http://www.ni.com/white-paper/2709/en/