Every screen has it's own resolution. Or rather pixel density.
Consider a 20" (diagonal) monitor:
A monitor set to 2560x1600px has a ppi of about 137
A monitor set to 1920x1080px has a ppi of about 102
A monitor set to 1440x900px has a ppi of about 89
I post "about" because actual physical size of the monitor is a factor as well. A 20" monitor with a 2560x1600 resolution will have a higher PPI than a 30" monitor at the same resolution. (This is the entire theory behind "retina" displays - huge resolutions on a small screen).
If you create a document at 300ppi and tell Acrobat to view it at 100%, Acrobat takes into account the monitor's pixels per inch and tries to match the monitor ppi to the document ppi. So a 300ppi document on a monitor with a pixel density of 100 would show 3 pixels for every 1 pixel of density. A 100ppi document on a 100 pixel dense monitor would be a 1:1 ratio. This is how Acrobat determines "100%".
If you want to accurately display sizes in Acrobat you need to alter Acrobat's preferences. If you go to Preferences > Page Display there is a field to input Custom Resolution. Input your monitor's PPI in that field and 100% will be much closer to actual size regardless of what a document PPI is. You'll also see an option in Page Display preferences to simply use the system settings. You can use that as well if it is reading a value. Not all systems will have a default value other than the standard 72/96ppi however.
For a close estimate at your monitor's pixel density you can use this calculator: Pixel Density Calculator
Note, the calculator will only get you close. It's not 100% accurate. For example it tells me my Monitor PPI is 100.63. However, using 100.63 in Acrobat causes all kinds of display errors. So I rounded to 101. If you want to be 100% accurate in the pixel density calculations you need to do the math yourself since every monitor manufacturer will have a slightly different variation for a monitor's size. The wiki article I linked to at the top of this answer explains the math behind pixel density.