info_search_time is the time it actually ran, search_now is when it was scheduled to run. You can sorta kinda see that by looking at info_search_time for addinfo (which is how some fields are generated) but there isn't much documented about the actual contents of the summary events. (I didn't go look at older versions of the docs to see if there was more before the si commands existed.)
Manually configure a search to populate a summary index
