For that matter, none of the HTTP streaming protocol are exact real time..
When user starts watching, he/she gets the latest playlist (m3u8) and any HLS client by specifications plays the third chunk from the last. So even if server presents latest 20 chunks, client will play with 3 chunk latency, not 20.
On FMS you can configure disk management duration which will keep required duration of the data on the disk. This data is not exactly the chunks understood by the HLS, but can be used to create them in realtime.