Adding Images to post – Image size

I am trying to add an image to my post, and everything goes smooth. The image is added, but even though it’s supposed to be 100%, it’s smaller and I have to adjust the size.

The image is 620px wide, and when I insert it it’s 490px. I guess if I select 100% it should be 620px without a question. The uploaded file has proper size.

Maybe there is something I am doing wrong or some “checkbox” that I’ve missed? Any help would be greatly appreciated.