I'm pretty sure it's just the physics. I started to explain it, but I think to just get you to think about it. You're a smart enough guy and you can figure it out if you want to. If you think that the diameter of the tube is a factor, then tell me why the figures never give that dimension. Why is the barometric pressure only given in mmHg or inH2O (the height of the column) with no consideration for the diameter of the tube?
The diameter of the tubes will affect how quickly change in pressure will register, just as adding a restriction in the tubing will do the same thing. Once a given pressure is reached, the height of the column will be the same regardless assuming a properly functioning setup ie. there's enough fluid to register the change, enough length of tube, etc.
I really don't want to be a smart ass, but we could get into the math if you'd like.
I did the math, its what I do for a living.
Not trying to be a s/a here either, but this isn't regestering with you what I have said,
Mercury manometers are historically defined with fixed dimensions... you can do some research and find what I have said.
With regard to plastic hose, used with atf, to show differential vacuume, it works, but it only shows balance. In order t register balance using atf, with a similar amount of definitive accuracy to ascertain viable "comparisons" of difference, the plastic tube needs to be much smaller in I.d. than 1/4" to effect the relative scaling factor..I'm doing this response on my nook, and have no desire to spend time playing to fully define right now, maybe tomorrow.