I have been trying to think of an easy way to explane what your seeing, so here it go's.
DVD are a compressed format, the compresion ratio is vairable, much the the way the jpg picture fromat is.
So a DVD set for high quality and get 1 hour of recoring time which is full 480p, or set it for low quality and get 6 hours of recording time, which is about the same quality as VHS. Most DVDs are some where in between the two. This varies from DVD to DVD.
Now the HDTV is in 720p or 1080i and so there is far more picture detail than your TV can display, but it is displaying eveything it can.
Since EDTV is 640x480 you would be upscalling DVDs then your EDTV would be down scalling the image back to 480p, it may make it look worse. John