Changing the data logging rate on a Tektronix DSO
This is what I found concerning your scope...

http://www.physics.rutgers.edu/ugrad/389/scope-TDS%202000%20%20Manual.pdf

In it, they go into the sampling rate's change based in your scaling. While you are changing the timing you see, it isn't going to continue sampling very far beyond your window. This allows it to get better resolution by scaling back. See the attached image for an example...