The question pretty much says it all. It's probably a ridiculously easy one to answer. It's cool outside but I know that my house is heating up because my AC kicks on periodically. I've had this experience in multiple houses, so I presume it's not just caused by some sort of one-off issue with an AC or thermostat. It seems strange and I'd like to understand why it happens so that perhaps in the future I can make my house more energy efficient.
I'm a life-long Florida resident and have had this experience in a few houses. I've noticed that even when it is cooler outside than in (say the AC is set to 80°F (27°C) and the weather is 76-78°F (24–26°C) outside), my AC still runs periodically throughout the day. Note that the obvious answer is "Just open the windows", but for those not familiar it is usually 80%-90% humidity outside, and even 76°F (24°C) isn't pleasant at 80% humidity (it's also a recipe for mold, which has been a problem for me). Basically, there are only 1 or 2 months of the year when it is both cool and dry enough to open your windows in Florida (IMO).
In that sense having the AC run periodically is not actually crazy, mainly just to keep the humidity down. However, it still seems strange to me that if the AC should kick on (aka the house is warming up) at all if it is 2-5°F (1-3°C) cooler outside than the temperature I have the AC set to.
I realize this is likely house-dependent, but I've lived in more or less the same "kind" of house for the past decade or so: wood frame, insulation-in-attic, single pane windows, and a few trees around providing shade for the house (although certainly not enough to shade the entire roof).
I presume this is a sign of some general inefficiencies in my house's thermal-insulation-design. Why might my house be heating up even when its cool outside?