How reliable is rear headroom spec?

[ktlimq]

Rear headroom catalog specs.

My 1992 Mercedes 300D with sunroof: 36.9 inches
2005 Mercedes C320 with sunroof: 37.3 inches
Mazda 3 4 door sedan without sunroof: 37.4 inches

When I sit on the rear seat, I feel that my 300D has the most head room among these three:

MB 300D > MB C320 > Mazda 3

Because my old seat pan cushion sagged down?
But I feel a little more headroom in MB C320 than in Mazda 3, while the catalog spec says Mazda 3 has more headroom than MB C320.


By the way, the Mazda 3 I drove had bad front seats. Only an hour of highway driving gave pain on my hip.
Mazda 5 front seat, however, was very good. The seat, pedal, and steering position could be adjusted so that the driving position is very comfortable for me.