Recently I encountered a problem where I could not create a cut-extrude feature. There's probably like, many such questions relating to this, but I still can't find an answer. The feature wouldn't compile, and gave me the error: "This sketch is an open contour." Here is a diagram of my sketch that had a problem extruding. 

I used the Check Sketch option to check for errors for my specific feature, however, no gaps were found at all. 

Afterwards, I tried to recreate this problem on a new sketch. The problem still lingered. What is different, though, is when I cut off the edges that do not form part of my contour, as such:

This was the exact shape that I used, with the parallel lines and two splines joining their vertices. However, the extrudes for this sketch compiled successfully.

Is there a logical explanation for this behaviour? Shouldn't Solidworks be able to identify the different closed contours within a sketch?

Thank you for reading my question.