I have hot water radiators, which are much slower than steam. The hot water is 70 degrees cooler than steam, and it takes longer to heat up all the water circulating through the system, which in turn has to heat up the radiators before they start heating the house, than it takes for steam to start heating up the radiators.

I'll grant you 8 degrees is an exception -- mostly when a warm-ish day where the system hasn't had to run very much turns to a very cold and very windy night. But a 4-5 degree swing is very typical and definitely noticeable, especially when trying to sleep through the "too hot" part of the swing.