I am currently under the understanding the Stack and reach are meerly a measurement of height and distance from the BB. So TT length, HT length, and their corresponding angles are the major determinations of fit along with BB height.
If I were to take a look at a couple of the most popular 54cm bikes, say the Felt B2, and the P2C. Their numbers vary a little bit in their geometry charts as published, the B2 having a lower BB matched to a lower HT. And as expected their stack and reach data is very similar.
However, looking at Trek’s 54cm data, with a 115 HT, their stack is significantly higher, about 2 CM. This in my mind can’t be all in extra BB drop, can it? Where is the rest of the stack difference coming from?
Probably a combination of head-tube length, head-tube angle (a steeper headtube angle will result in more stack as more of the headtube is taken up in vertical distance), fork crown height - all the bikes have different forks and the distance from the brake mount to where it meets the frame can be quite a bit different on different forks - especially proprietary designs, and bottom bracket drop. If you get 5mm from headtube height (as you do with the TTX vs. the Felt), 1mm from headtube angle, 1cm of bb drop, and 4mm of difference in crown height, that would be 2cm of more stack (those are just made up numbers) without any of the specific geometric measurements being markedly different. So that would be an example of how you could get those differences. Which is why stack and reach are so valuable as opposed to looking at just TT length, etc. The other very important thing to consider is the seatpost, since if you want to ride at angle X, you need to be sure you can get your saddle to that angle.
I had forgot about the crown height. Though, I would be surprised to find the Trek had 1 full cm more bb drop than the Felt. At 8 cm thats a fairly low bottom bracket.
Thanks for the insight Rapp.