Tomas,
If you are working in RGB, you may forget about PPI and just use Save for Web, setting the actual pixel x pixel size in the Image Size window (where you can also set Art Optimized).
That being said, obviously it should be possible to export at the right size/PPI.
But how are you determining the size?
If you try to check it in Illy, you may have it shown at 72 PPI, more than 4 times larger, or something like that.