Non-Anamorphic DVD's

[Lynos]

Hi folks

So... I've been looking to get some old movies on DVD that never came out on Blu-Ray. For some reason, some of the DVD's that are out there are anamorphic and some aren't, even ones that came out late in the DVD cycle. For example, looking at DVDCOMPARE for The Roaring Twenties, it says the DVD is non-anamorphic (this information btw is not found not on BR.com, not on Amazon, and not on the back of the packaging).
Now, I have a few non-anamorphic DVD's in my collection that play fine on a 16:9 TV, so no black bars all around and image the size of a large stamp. I also had some non-anamorphic DVD's that did look like a stamp and I got rid of those.

So I am confused. How come some non-anamorphic DVD's look fine and some don't, and how can I be sure that when I buy a DVD it will look as it should on my 16:9 TV?

Any thoughts are welcome.

Thanks!