It lists both here. Because both are commonly used, and thus "possible" (whatever that exactly means in this context). Leaving out okurigana like these that appear inside a word is a common thing, and you might not always find them spelled in a dictionary in the same way as someone chose to write a word in real life. That doesn't make them spelling mistakes really, just harder to read if you're not familiar with a certain word.
Both versions are here on the entry you are referring to, so doesn't that address it?
This kind of spelling is often seen on boards, doors etc. For example, 立ち入り禁止 is correct, but on boards you will see this spelling: 立入禁止
It consumes less space.
Spelling mistake or not?
In class we learn 入り口, but here it's just 入口, where the -り is included in the 入. So are both possible, or is it just that 入口 is so commonly written that way, that they just use that?