Tires have a property called "Conicity" (Root word: Cone). It's the force pushing sideways. It varies from tire to tire and can push in either direction - that is towards the center of the vehicle or towards the outside. You vector subtract the values to get the net amount of force pushing sideways.
Each vehicles has a certain sensitivity to conicity - a value above which the vehicle starts to drift or pull. Below that value, the steering system has enough internal friction to prevent the drift or pull.
So you can have a pair of front tires where the difference in the forces is larger than the sensitivity of the vehicle and you'll get a pull. Swapping the tires left to right should reverse the pull.
BTW, you can wear conicity into a tire by misalignment.
- BUT -
Alignment can also cause a pull. So to see if it is the tires or the alignment, swap the front tires left to right:
1) If the pull completely changes direction, then the problem is 100% in the tires. You'll have to either change the tires or live with it. (You could also swap the tires around to find a position where this doesn't happen, but you will have problems rotating tires.)
2) If the pull doesn't change at all, then it's 100% in the vehicle. Usually alignment, but a dragging brake can also do this.
3) If the pull disappears, then it is both the tires and the alignment. In all likelihood, the tires are going to wear funny, so this isn't the best solution.
