Assume the lines each have one hundred cars in them. If you shuffle merge at the last point, everything is even.
But you, being the eager beaver that you are, moved over somewhere a mile before that point. Now line 1 has 101 cars in it, and line 2 has only 99. Because you’ve been in the line for awhile, when you get to the front, you have to let someone from line 2 shuffle merge in front of you, meaning that two cars got over from 1 to 2 in the space where only one should have. Plus, though, the guy who let you in now has to let another person in. That’s three! Three cars where only one would otherwise have merged.
It doesn’t take a lot of early movers before lane 2 is emptying out faster than water bucket at the OK corral.