Well actually........... celtic_druid.
720/2.35 does equal 306.4. However if you applied the same formula to calculate the aspect ratio's for 1.85 & 1.77 you would end up with 389.2 and 405 respectively. Which is way off the mark.
If you live in say a PAL country and watch a true 16:9 (1.77:1) DVD on a 16:9 widescreen TV or PC monitor. Provided you have your DVD player software (and/or hardware) configured correctly, you will find that the resulting image will completely fill the 720x576 pixel frame, not just 720x405 pixels.
This is why many people get confused when setting up their encoders and end up using all sorts of filters etc to make encoded images look better.
Copying DVD images is a bit tricky because all 'widescreen' DVD's are formatted to fit onto/over a 720x576 or 720x480 background. DVD's are manufactured like this so they can be viewed, at their best, on a 16:9 (1.77:1) widescreen TV.
This is why when if you view DVD's on a 4:3 (1.33:1) TV, via a stand alone DVD player. You have to reconfigure the DVD players output (using the DVD players own software) to tell it that it's hooked up to a 4:3 (1.33:1) TV.
Unfortunately, PC DVD playback software is (in the main) configured for 4:3 PC monitors and as such causes confusion during encoding especially to newbies.
I've been reconfiguring video frame sizes (in telecine) for years.
It's a real shame that there are so many encoder applications out there. Each one has very different ways of inputting the source image. And if the source ain't handled right, the resulting encode ain't right!
I really must write some info about this....... happy encoding everyone.