Why does Bootstrap 3 force the container width to certain sizes?

Question

If my browser is sized to be 992px wide, then .container has a max-width of 970px. If my browser is sized to be 991px wide, then .container has a max-width of 750px. Why is

Answer 1

This is because fixed-width + margin = break point. In bootstrap the margin grows while the container remains at a fixed width. When the browser is res-sized past break-point the container jumps just below break-point.