They say; "There are many factors other than string gauge that determine the actual and PERCEIVED string tension on your instrument". The flexibility of the instruments top and neck and neck relief don't affect the tension, only the perceived tension.
Weather and temperature can both affect tension--the tuning will change, i.e. a change in tension, with more or less humidity and/or heat unless the top is very, very stiff--like a laminated top. A high quality, well built instrument with thin top plates and light bracing will move (tuning drops or rises) more with humidity and will even change with body heat. I have a couple of instruments that always need re-tuning after I hold them close for a while--they absorb body heat. The tension change may not be much but it's there.
The string tension in the tuned or quiescent state is what ever the tuning is. I assume that D'addario intended this statement to mean that once the guitar is being played that the string tension and subsequent feel and response will not be the same for all instruments.
Any deviation from this balanced state by way of applying a guitarist to the equation will generally result in an increase in tension of the strings. The degree of this increase affecting the feel of the stings under playing conditions will be related to the flexibility of the soundboard (and some other things not related to answering this question).. Its not a linear equation because their is "give" in wood under stress and there are changing angles and leverages involved as the soundboard deviates from its normal orientation.
A few other variables affect this and no doubt they will be teased out in subsequent posts - which is valuable because it helps understand (and helps me understand) that vexing problem of why some instruments feel so good to play while others seem to fight against you.
Good subject, good questions,
Thanks everyone for your useful comments.
Like Rusell pointed out. I too am trying to understand why you can have two identical guitars with the same specs such as
the same string break-angle or length at the nut and saddle/bridge, length of the string between the tuning posts,gauge of strings,action height etc etc, and one guitar can feel great and the other 'stiff'.
I understand it's also such a subjective thing.
What may feel stiff to one player may feel great to the other.