It's locale sensitivity.

Duck doesn't mean very much in Germany, so there's little that competes with duck.com. In Britain duck has many other meanings.

If you search for an abbreviation that's commonly used as company name, you may find that Google's top result is the one near you, and that the eponymous company in Petrapavlovsk isn't listed.

I doubt it's a bug. I think we just eat a lot of duck in the UK. Additionally I've never heard of duckduckgo referred to by just duck