Not having seen the settings, acquisition and all that -- it could easily just be a glitch, with sinc interpolation making it look believable. (The suspiciously small number of points seems to hint at that?)
I can do the same thing, to literally unbelievable risetimes, on my TDS460 -- this being relatively easy as it uses equivalent time sampling. (Heh, though I don't know how I'd trick it to acquire a nice step; it's random sampling I believe, it just assembles a waveform bit by bit.)
Heh, and at maximum settings (1ns/div sweep, 50x zoom), it can show all the way down to 20ps/div. Not that anything it can properly acquire is at all meaningful there (it's only 350MHz BW).
Tim