The “since” parameter is a record of the event log that you would have received in the defined time period, not the pages changed. The fact that you see a date modified from the previous day is likely because that particular page (or a set of pages) was reverted to a past version of the page, which will show up as a new event/page to handle accordingly. As a general rule, we recommend tracking “dt” somewhere in your system when working with the Realtime streaming endpoints so you can pick up where you left off in terms of the event log.