This question is AWESOME! ... I think we're all getting too technical though.
100 x 100 pixel image = 10000 total pixels
Scaling an image down pulls pixels out. Scaling up adds them. Either way the software takes an "educated guess" as to alter the file.
A single reduction: 90 x 90 (1900px removed from the original file information)
2 Step reduction: 95 x 95 (975px removed), 90 x 90 (another 925). The detail to catch here is that of the total 1900px removed - 975 of them were NOT part of the original information.
The original image is always the best. Fewer "generations" always equates to better quality (closest to the original quality).