Unwrap can as stripes anybody could explain me why the top part of this can doesn't unwrap as a stripe?
unlike the middle and bottom sections?

here is the blend file: https://drive.google.com/file/d/0B2TovTwIWuC7ZFFVMXBSQjhBX0k/view?usp=sharing
From your image it looks like you have overlapping loops at the top of your cylinder, you can see that because of the face indicators appearing in the middle of the edges, and thats why you could not select the hidden edge to mark the seam and have a proper unwrap.