In reply to Mike Hartley: Some of the routes are overgraded, some aren't, and a few are undergraded. All depends on the style of the route and what your strengths and weaknesses are.
I've not found kendal wall grades to be any more or less skewif than other climbing walls.
It's a climbing wall, just judge the grades relative to your own progress, because anywhere you climb outside is going to be different anyways.
And maybe it was just because you were tired?