The marquee Selector doesn't select the right area

Hello there!
This has been happening since before Photoshop Beta arrived and so far it hasn't been fixed. Whenever I select an area with the Retangular Marquee Selector it adds an extra pixel to the edge of the cut. For example, if I need to divide an area of 3240X1080 pixels into 3 equal parts, when I use the tool, with or without the help of the guide lines, I never get 3 equal parts of 1080X1080px. The first part always appears at 1081X1080px, the second part at 1082X1080px and the third part at 1081X1080. The tool always adds 1 extra pixel to the side that is cut off. What is the solution to this problem? It doesn't happen in Photoshop Beta.