A few years ago the POH was being used as a sales aid.
I think you hit the nail on the head there.
Aircraft from the 60-ies to the 80-ies in the POH tend to have some inflated numbers under more then ideal conditions or conveniently in MPH iso KTS (20% difference there).
At least some of the new aircraft that I fly have somewhat pessimistic performance figures and you do not need exceptional pilot skill to match them.
So old, unreliable numbers, new pessimistic.
Also remember that an aircraft tends to gain weight over time.
Dirt accumulates in nooks and crannies and repainting the aircraft without removing all of the old paint increases weight.
Major body panels will be replaced, cowlings, wing sheet metal, tail sections.
All depending on the amount of hangar rash or fender benders that the aircraft goes through. This will ususally lead to the aircraft being slightly out of rig which will cost a mile or two on the top end.
10 little items that will cost you a mph each add up.
A good example is the reverse, you can make a Pa28 fixed gear or an Arrow go 10-15 kts faster in cruise.
By adding so called speed mods or speed kits. The difference is usually very small per component, 2 kts for better wheel fairings, 2 kts for aileron seals, 2 kts for flap seals, 1 knot for wing fairing, etc etc.
Will cost about $1000 per knot speed increase by the way.
But you see how the process works both ways.