I don't know if any of you have ever experienced this problem

This is about me, my PC and my VBA emulator

Sometimes (actually not just sometimes, once it happened I have to restart my programs) when I take a screen capture of my emulation, and paste it to MS Paint, the pixel size suddenly changes without a cause!
I'll try to explain it:
VBA ROMs are run at resolution 240x160 pixels-> this is proven everytime I check my screen captures; the image file is said to have 240x160 pixel size. But when I paste it to MS Paint, suddenly the picture gets 'corrupted' and becomes 240x159 resolution, even though I cut and paste the whole area of the image (which is said to be 240x160).
If you know why, please reply. Thanks