I thought this might be a time range bug, but I tested locally and it is not. Seems like a document count issue, due to the way that list is populated. Since that issue is still open, those values are likely still hard-coded and nothing short of a custom Kibana build will let you change them.
Looking at that issue, it seems like a number of users are running into that problem. I don't know if the team that maintains that is actively working on a fix or not, but https://github.com/elastic/kibana/issues/17388 is where to watch to find out, ask questions, and otherwise discuss it.
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