why is this oscilloscope so cheap with a 20Gsa/s sampling resolution?
This is for a repetitive signal like a standing sine wave. 

When I buy a scope I always look for is the real-time sample rate which indicates the maximum rate an incoming signal can be acquired in a single-shot acquisition. On the other hand, repetitive sample is a method of reconstructing a signal based on a series of triggered waveforms that are each acquired in single-shot mode. The advantage of repetitive mode is that it offers a higher effective sample rate. The downside, however, is that it takes more time and is applicable only for repetitive signals. This scope has a real time rate of only 200MSa/s - far from the 20GSa/s.

In general I hate using PC based oscilloscopes and would tell you to go for something like this instead if you're in the market for a scope to use. Unless you are really strapped for money and need the 200MHz bandwidth or really need the deep sample rate.