In Swedish, it's a bit unclear what ägg is in this case. Presumably it's plural, which is why eggs is the preferred solution. But it is also possible that it could be a mass noun, which is why we also accept the answer egg.
In English, different speakers have different opinions about whether egg can be treated as a mass noun or not. The main divide seems to coincide with the Atlantic ocean, at least according to this blog. We try to accept both American and British English in this course (the main sentences should reflect American usage, but both should be accepted).
If you want to say a sandwich with an egg in English, that would definitely be en smörgås med ett ägg in Swedish too, so that answer cannot be accepted.
And the reason …with egg is shown to you is that the system tries to match whatever you input to whatever it thinks is the closest accepted answer.