What origin are these x and y coordinates based off of?
These x and y coordinates are based off of the registration point of tab button.
43 != 29
When you work with symbols on a stage, the symbols that you're working with aren't the actual original Library symbol. They're copies that can be manipulated by scaling them, applying color and opacity effects and...
This instance of tab button is scaled, if you open library panel and edit tab symbol you can see the actual size.
UPDATE
after I change width and height of tab button to 19.80:
29*(150/100)=43.5