The route by Witches Stride is along a old railway line - do you think it will be steep?
Look at the contours there - if the plot is out by the merest smidge you will either be half way up a cliff or halfway down the embankment! Even more likely since your plot is a straight line between WP30 & 31 so it clips the cliff!
The second point is the accuracy of the source map you were using, how wide is a railway track in reality and how wide is it on the map if you apply the scale? Now throw in the underlying elevation data which is not continuous and even if the point were bang on on the map, as drawn, the elevation data could still be out.
Sometimes you have to use a bit of common sense when interpreting a profile. Let me give you an example - the SWCP runs along the edge of some fairly high cliffs. Therefore the line between waypoints could pass over the sea which would show a fairly steep climb! Even if you were to lay a live gps trace onto the map - the elevation profile can be gibberish, a 15ft error in a fix could put you 50 feet higher or lower than you actually were.
Points to take away - maps are a small flat representation of a large and complex 3D surface with all the limitations inherent therein.
Gps maps and data are not infallible either.
Common sense counts for a lot.
One last thing, look at your route between WPs 11,12 & 13, see the pecked line cutting the corner by the field boundary? Which way do you think it is most likely to go?
In this case experience says it could be either since the green line wiggles but if it were straight, my money would be on the pecked line