I would say because when you are creating it on MS paint, the original format is a lossless format, something like the MS paint file or a BMP file or the PSD file in photoshop. And i'm pretty sure you are using a jpeg which is a lossy format, meaning the quality is lowered to save size. Since it's a digital format instead of those magnetic VHS tape which cannot transfer data in a 100% ratio, a digital transfer of data should hold whatever data it is uploaded as.
You should save it as a BMP format and a jpeg to test out what's the different b/t them. Of course this is assuming you saved it as jpeg.
Here's an interesting article on jpeg and it's compression from wiki -
http://en.wikipedia.org/wiki/JPEG
from about.com -
http://pcworld.about.com/news/Dec072004id118543.htm