css - two inline-block, width 50% elements don't stack

I would like to have two columns of 50% width space, and avoid floats.
So i thought using display:inline-block.

When the elements add to 99% width (eg 50%, 49%, http://jsfiddle.net/XCDsu/2/ ) it works as expected.

When using 100% width (eg 50%, 50%, http://jsfiddle.net/XCDsu/3/ ) the second column breaks to the second line.

Why is that?

---