llGetAgentHeight is for your avatar's eye-height when firing weapons in Mouselook. So, to start, it's not your avatar height at all. But, all "Height Scripts" us it and embellish the value with an offset that, to date, almost nobody agrees upon. ()
Add to that, the default avatars have always been disproportionately tall. So, add this debated offset to an already offset sense of scale and you have a whole new set of "corrections" of people defining "real" height. ()
Add to that, the camera leads to most indoors locations having very high ceilings... Further diminishing people's sense of visual scale and shifting opinions on what "real" height is. ()
Add to that, the oneupsmanship of people wanting to be taller and more intimidating than everyone else... Like a virtual height arms race, people would inch their sliders closer and closer to 100, just to be the bigger, badder, moar beauturphull avatar. (>_<)
... And, you get where we're at now. =^-^=
Add to that... Because of this mess, we have people honestly saying "A meter is not a meter here" with a straight face. Even though Havok4 definitions of gravity and force are based on the meter. People have decided that 1 does not equal 1. ()
But, all isn't lost. The Emerald client does have accurate meter-based height measurements in the avatar appearance dialogue. And, more and more people are doing as you've done and actually visually referring to prims in order to determine the height of things. ()
In the end, it's all relative. You determine what you want to be. But, don't be surprized when someone calls you out if you claim to be 5' tall and actually measure up to 7'3" prim height. ()
I've gone as far as defining it in my estate covenant. =^-^=
This sim is to be constructed to-scale. Meaning the meter will be honored as-is and "short" avatars will be considered proper adult height. Avatars taller than 1.8 meters in real prim height will be considered creatures of fantasy and lore and will NOT be used as reference in order to falsely accuse properly scaled avatars of being depictions of children.
That said; I'm 5'1". ()y