You are right that everyone should be working to the same standards and understanding, but that is not the case.
Only recently I was working a stream of inbounds to the LTMA from the east. I had told every aircraft joining the frequency that the delay was approx 10 mins and suggested a suitable conversion speed which kept the stream apart but also didn't push them into the LTMA at 300kts or more (not sure I agree with anotherthing's scenario of 300kts into the hold for this reason!)
One aircrew from a certain major UK carrier based at Heathrow reported the actual indicated airspeed as 270kts. Since he was at the head of that bunch of traffic but some miles behind the previous bunch, I requested him to fly not less than 270kts. Imagine my surprise to see him accelerate and threaten to catch up the bunch in front. When questioned, he reported his indicated as 325kts! This into a 10 minute hold that he had been told about and was an increase of 55kts from the speed he had previously been flying!
Unfortunately we have to deal with the lowest common denominator, and some of the reasons already given may help to explain why a speed reduction may not be a good idea in the descent.
However, as with all things ATC, if you need clarification at the time: ask!