When tabs act like that, it's often because one is using tabs two different ways at the same time.
If you don't put tab stops in the ruler, the tabs default to a certain number of spaces.
If you use the tab stops, the tabs will align accordingly.
If you use the tab stops but put in extra tab characters, things don't appear as expected.
When setting things up with tabs, I always display formatting marks so I can see where the tab characters are (the little arrows).
The other thing that can affect tabs is where the right and left margins are, but if you display formatting all tab behavior should be, well, well behaved.