I too have seen this a handful of times and thought, "Wow, that's really clever Siri!" only to realize a few seconds later that Oct 21st vs Oct 20th does not help me and I am still screwed. Then I cancel out and go look at the calendar day and then re-sirify it.
Then again I am not that smart.
It's showing you the next two occurrences of the time you requested and asking if you want to be notified on the next occurrence of that time or the following one.