Translation:The blankets are warm and comfortable.
Sheets are thin and usually made of cotton. There are usually two. One covers the mattress and the other is put directly on top - to keep your body from touching the other bedding. We sleep "between the sheets." On top of the sheets, we then put various covers and/or blankets. Finally, you can cover all that with a decorative "bed spread" which is usually not intended to keep you warm on anything but the more mild nights.
I suspect that this is a quirk of the Duolingo system and not of the Duolingo Esperanto Course. The course authors can control which alternate translations are included in the course, but they can't control which answer you are shown. The "should-have" answer is chosen by the system based on your reply. It's likely you had a typo or unusual wording in your response and the system looked for the one which it thought was closest to your answer.
As a human teacher of Esperanto, I will tell you that you "should" have put warm - as indicated in the OP of this thread.
This is just contextual. I think of "varma" as meaning "warmer than it usually is." So, "la kafo estas varma" means "the coffee is hot" because coffee usually is warm. Blankets, however, are usually room temperature, so "la litkovrilojn estas varmaj" means "the blankets are warm," because they're just warmer than usual.
Sorry if I confused you.
This explanation seems unnecessarily confusing.
One problem is that the word "varma" looks like "warm" (and was taken, way back when, from the English word "warm") but the primary meaning actually is hot.
"la kafo estas varma" means the coffee is hot. Punkto , fino.
Warm blankets are not warm because of their temperature. They're warm because of their qualities. A warm blanket will keep you warm.
Yes, sometimes "varma" will be translated "warm" - but this is more due to a quirk in English.
- Varma kafo - coffee with a high temperature
- Varma litkovrilo - a blanket that helps preserve your high temperature.
We have blankets which are never used on a bed... but they're still blankets. I've also seen decorative quilts, and "lap robes". I would say litkovrilo for anything which could function for keeping people warm when spread over a bed - this includes the fleece "emergency blanket" I have which was designed to be rolled up and stored in a car. (I do often use it while camping even though I often don't have a bed to cover.)
Esperanto words aren't definitions. They're meant to evoke a meaning.
Reminds me of something which came to mind recently. A real thing that I said in Esperanto.
- La kuierejo ne estas havebla ĉar la krudmanĝantoj bezonas ĝin por prepari sian manĝaĵon.
- The kitchen is not available because the raw-food-ists need it to prepare their food.
So, they need a "cooking place" to prepare their raw food.
It's also possible to say "Li klarigis, sed ne tre klare."
You will eventually need to learn to express ideas and not just translate words. The primary meaning of varma is "hot like fire." It has other meanings -- including "having the qualities that protect you from the cold". English requires you to say "hot" for one and "warm" for the other.