What they're referring to is that analogue CRTs don't really have a fixed horizontal resolution. The screen has a finite number of horizontal lines (i.e. rows) which it moves down through on a regular-timed basis, but as the beam scans across horizontally it can basically be continuous (limited by the signal and the radius of the beam). This is why screen resolutions are referred to by their vertical resolutions alone (e.g. 360p = 360 lines, progressive scan [as opposed to interlaced]).

I'm probably wrong on the specifics, but that gives the gist and enough keywords to find a better explanation.

[EDIT: A word.]