Yes and no. It's part of the reasons.

Another one is a lot of websites would only test on Chrome. So if Chrome implemented a standard in a slightly different way than the spec, it would break on all other browsers.

But well-designed websites don't alter their behavior based on the user agent string. They test for capabilities before using them.

Indeed.

That's the correct thing to do and it's what real software engineers will do. But from the horrors I've seen in offshored websites, that's science-fiction to some coders. They will assume whatever version of Chrome they are running is the only one in existence and will use user-agent string once someone tells them their site is broken in Safari.