Would be interesting to take a thermometer on the plane.
I do. With the low humidity, it feels cooler than it is. Talk here of 21 being a good temperature is amusing. It almost never happens. The temp has dipped below 21 on 4 of my last 100 flights. (Three times to 20.9 and once to 20.5. AA, BR, FJ and VA, if anyone cares.) Average temp for most airlines is around 23.5-24.
Most galleys are also not heated (for reasons unknown to me), so when the cabin crew are hiding away in the galleys they tend to heat up the cabin (if they can) or ask us to.
On ANA, one of the worst in this regard, I decided to just stand next to the lovely cool galley one time. When an FA eventually asked why I was standing there, I replied that because the heat was so high in the cabin, this was the only comfortable place on the plane. They actually turned the heat down then.
Another bad one is Asiana. Last time I was on one of their flights, I asked if the cabin temp could be turned down and nothing changed. When the head FA came by, I asked again. She sounded uncertain, so I told her I had another way to be comfortable then and poured the rest of my iced water down my back. The shocked look on her face was priceless and the cabin cooled down about a degree after that.
When travelling on ANA the temperature had been set at 23 degrees, and when queried the flight attendants confirmed it was because Japanese passengers preferred the cabin to be warmer - and this was in the back of economy. Still fly them as their service is normally impeccable.
On the 763, NH would sometimes have the cabin temp around 27. I was once on a stifling NRT-HKG flight with only a handful of passengers, so I wandered to every section of the plane. It was the same (26.5+) everywhere. These days on the 788, it tends to average about 25 in my experience.