As its summer, have you been running your AC more? Or maybe some ceiling or wall fans?
Quoting this tech article
Always On is a calculation of the lowest power of each of your mains, added together, where “lowest” refers to the 1% bin of the observed wattage histogram over the previous 24-48 hour period. It is updated every half second, though most users will not see significant real-time changes given the 48 hour lookback window.
I quote this article because long running devices can be captured by Always On, but then if said device is turned off, it takes a while for Always On to catch up.
As an example
If you turn on 100w ceiling fan: When you first turn it on, that 100w will go into Unknown (unless Sense has detected your fan and then it would be whatever the device name is". After around 12-24-48 hours, it will shift into Always On as its been on for such a long time that Sense has now determined that there is a new “low water mark” for power in your house.
If you turn off that 100w fan, you won’t see Always On drop right away as the Always On calculation is looking at the last 48 hours to determine the Always On calculation. So the fan would then need to be off long enough that Sense no longer considers it always on.
So if it has been really hot and your AC, or at least blowers, have been on for significantly longer times, its possible they are being captured by the always on. But then you have a cool day and they run less / turn off and Always On re calculated a day later.
Check out this thread as well for another write up of Always On.