Copy and export picture VBA - pixels issue

Hi,

I'm actually using chart.export command to get some .gif files.
The problem is the resolution, this is my case:

Chart width = 480 points
Screen Dpi = 96
Chart width should in pixels = 640 (480 x 96/72)

The problem is that the output file is 643 width. If I test the same file in another PC, the output file is 643 width.

Does any one know how to solve this??

Many thanks!