Orbits are elliptical, AFAIK, and the two lengths are the dimensions of the major and minor axis of the ellipsis. The planned operational orbit was circular (750 x 750 km).
Sorry I deleted my comment because I had already discovered the answer.
(I asked what the meaning was when an orbit is written as XXXxYYYkm.)
From my reading of both articles together, I'm pretty you're correct - it's the interval that the satellite's altitude moves between (ie the axis's on the ellipse around the Earth, like you say)
