I did some playing around last night with my 100 MHz scope, 150 MHz probes, along with 4 MHz, 20 MHz and 40 MHz square waves. And as expected at 40 MHz they didn't look like a square wave at all. But one big difference from the OP's screen-grabs was that it looked nothing like a pure sine.
So I guess that the making a 100 MHz scope hardware into a 50 MHz scope is achieved through a brick wall 50MHz DSP filter, hence the OP's 'pure' looking sine? If that is the case, then that is a reasonable explanation for his traces...
This could be verified by watching the waveform as the frequency is swept. Between 15 MHz and 18 MHz the waveform should change dramatically as the 3rd harmonic gets nuked by the filter, and between 9.5 MHz and 10.5 Mhz when the 5th harmonic gets cut.
Between 10.5MHz and 15MHz the waveform should stay pretty much the same shape.