You are comparing three different things.
So they have three different URLS
Additionally VOD Thumbnails can and will change. Recent VOD’s tend to have one URL and older VOD’s have a different URL.
If you think this should be changed of you have a feature request.
https://twitch.uservoice.com/forums/310213/
Personally I don’t have a problem I fetch and display whatever it is from the JSON responses from the API without issue. So if it changes then nothing breaks. And I don’t have to consider about constructing it from a fixed format if I can just fetch it