The glide angle of a stopped engine is steeper than the idling engine. Is anyone really suprised at that fact? The problem with that is in the execution of the procedure for real. The standard PFL pattern doesn't give much lattitude for correction, so when the it's discovered that the angle is steeper in the real case - it's often too late and the pilot ends up in trouble.
I grew up using the military 'Contant Sight Line Angle' (SLA) technique which gives you a lot more lattitude and if your glide angle is steeper, the technique will allow you to identify that fact quickly and correct it with plenty of room to adjust the pattern to compensate. When I trained as a pilot, I already had about 400 hours gliding. The PFL technique I learned is what I had used sucessfully a thousand times or more in gliders. And I have had cause to use the technique twice for real in powered aricraft - both time with complete success.
When I converted my military instructional qualifications into the civilian rating, I was required to teach the PPl technique. I found I couldn't do it! Judging the glide angle was very difficult and I either over or undershot on every attempt. After some scorn from my instructor, I then demonstrated three glide circuits using the constant SLA landing on the numbers each time. I subsequently had to learn the PPl technique, but I will never be convinced that it is better than the SLA technique.
So why isn't the SLA technique tought in the PPL syllabus? This question was being banded around in the 90's when I was an RAF QFI on piston singles. The CAA safety magazine had a discussion for nearly a year regarding this question and some interesting facts emerged. The Statistics showed that military pilots had a forced landing sucess rate of over 90% whereas the civilan sucess rate was less than 50%. This is part due to the mlitary pilots being in better practice, but a lot of it was considered to be due to the SLA technique. There was a move to change the PPL syllabus to the SLA technique, but there is a lot of inertia in the system and it was considered the technique was too complex to learn and would take too many resources. this is slightly ironic as most pilots who have learned it subsequently confess it isn't difficult.
If your club has an ex military QFI, get him to demonstrate it one day.