I'll answer my own question (having read the FAQ...): likely the base map has changed since I plotted this route. In the FAQ you say this about route recalculation):

(This also applies to showing turn-by-turn directions and the elevation graph for an old route: in both cases the route will be recalculated.)

Andrew, if you're not aware we can add viapoints along a route to ensure that it doesn't change with adjustments to the underlying data or the algorithm. 

On top of that, if the original is saved as a gpx file we can overlay that as a 'trace' on a newer version to see where the changes occurred. 

Thanks. Today I tried making some minor alterations to the route, then re-saving it, and I could then view the elevation profile without any recalculation happening.