No, you (almost) had it right! 
I quote the launch announcement:
- Hourly aggregation is calculated every hour (the previous hour is computed)
- Daily aggregation is calculated every day at midnight (the previous day is computed)
- Monthly aggregation is calculated every 1st of the month month at midnight (the previous month is computed)
But the thing is, when you display the « last 24h » view, imagine you display that view at 6pm, the daily aggregation for the current day is not ready yet (it will be calculated at midnight), so we use the hourly aggregation which is available.
In my tests, I tried to make a mix by doing:
If you request the dashboard view at 6pm:
- Take from 6pm to midnight of the previous day via the daily aggregation
- Take from 00:00 to 5pm using the hourly aggregation
- Take from 5pm to 6pm by taking the live data
But it really doesn’t look good, the mixed data makes the « jumps » clearly visible on the chart; it’s really not clean.