Photoshop :: Different Selections From Same Logical Procedure (Mac OS 10.6.8)

Example:

Create or open a document.

Create a horizontal guide by dragging it from the ruler.
 
Now use the marquee tool to select the upper half of the picture, with the bottom of the selection snapping to the guide.
 
Result:Some times it happens that the resulting selection is different than the (logically) same selection made from the lower half of the picture to the guide and then inverted.Effectively this means sometimes when I use a snapping guide as the limit of a selection from one direction it is not border-to-border with one drawn from the other side. Then a line of one pixel height 'behind' the guide is not selected by either one of these procedures.
 
Expected Result:When the guide (which should have no width or height at all) forms the border of a selection from one side, it should be the inverse of a selection touching the guide from the other side.
 
System Information
 
Adobe Photoshop Version: 13.0 (13.0 20120315.r.428 2012/03/15:21:00:00) x64
Operating System: Mac OS 10.6.8
System architecture: Intel CPU Family:6, Model:30, Stepping:5 with MMX, SSE Integer, SSE FP, SSE2, SSE3, SSE4.1, SSE4.2
Physical processor count: 4
Processor speed: 2660 MHz
Built-in memory: 16384 MB

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