I see what you mean. Yes 20ps is really fast. I am sure that is usefull for some cases. The 33120A can never do that.

I checked what the 33120A could do with my SC504 scope, which is 80mHz. The 33120A can repeat the signal at 1mHz. I could verify the 0.1uS/div with it. 10 repetitions, and 1/5 of the last division too slow. So 2% too slow. I could not verify the fastest range 50ns/div any more. But at any lower range it is 2% too slow as well. So just when I make no mistake, with the 33120A I could see this 80mHz scope time base is 2% off.