Not on purpose.
It's just that when the course was created, most sentences only have a small subset of the possible range of translations that could occur. Expanding this number of translations per sentence is one reason for the beta period that the course is in.
Please help! When you come across a translation that was rejected because the maintainers hadn't thought of a particular turn of phrase or synonym yet, use the "Report a problem / my translation should be accepted" button (or the one with a flag item). Then you needn't post in the discussion threads, either - reporting is more effective at bringing missing alternatives to the attention of those who can actually make the required changes.
And please be patient. It's a bit difficult to roll out global changes (e.g. "accept large anywhere that currently accepts big") and so in general, changes are on a per-sentence level, so please report every sentence where a given word is not accepted because it wants a synonym, even if you had reported a similar sentence just the previous day.
And changes are not immediate, so it may be a few weeks between reporting a missing translation and that translation being added to that sentence.
At last it is September.WHY ON EARTH is this considered wrong by YOU.To add to it there is NO option to write My sentence should be accepted.There are other options about sound and other similar problems.YOU should have thought of this option before giving it to us.Is this yet another demonstration of the lack of thought while planning this course?