This is just a hunch but I would think about this issue from the employer's point of view. My guess is that for example ILM is getting more applications than they really want. So it is obvious that only the best are selected for further interviews. The demo reel should reflect the absolute best of your capabilities and expertise. This is of course only my opinion. I don't have any experience whatsoever since I am in the process of putting together my first demo reel also.

This could be of course totally different situation for a small company who takes whoever they can to work for them