That it is truncated and random, as in, not the same every day and you do not know how it is ranked?
I do not know if staff or faq said it somewhere. I seem to remember the mod explaining it some time ago in response to a complaint that an indexed item could not be found by exact title search, but I do not find the thread among all the threads about games not indexed. Maybe I misremember the details. That staff will not tell how those things work exactly you can read several times.
But you can observe and test this for yourself. The capping of results is obvious. And if you look now and in a few days and will have a different ordering and different items you have proof that the results change. What you do not know, is the mechanism of the change. Is it pure rng. A cycling of a pool of available items. A combination of those and other factors, inlcuding popular ranking?
We do know that the search was recently overhauled and will be again. There is an announcement by leafo in developer updates subforum about this.
What I still do not really get, if the search is just optimized for title search or if it can only search/find title matches. I would expect it to not be a pure title search, but maybe it is. It certainly does not help that there are so many games on itch that for practically every word you will have games with that word in the title.