It may help to look at the raw results for more insights.
The heartbeat is checked once per minute. If you have the AMA and MMA agents on the same system you will see up to 2 per minute. That could explain 20 records in 10 minutes.
Due to ingestion delays and other latency impacts the entries appear to jump 3-4 minutes periodically. You may need to modify your query to produce more consistent results. The following article may help.