Summary
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.
More Details
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?
Best Answer
Meet solar gain
For the most vivid example of solar gain, sit in your car with everything off - A/C off, blower off, windows rolled up tight, doors shut tight. You can't. Your body will force you to open a door or something because it will become unbearably hot within minutes.
If it were always night, or if your house were entirely in shade, this would not be an issue. However, your house is in sun, and that means it is being actively heated by about 100 watts (300 BTU/hr) per square foot (around 1 KW/m²).
That's really a square foot directly facing the sun, so it'll be less if you're not at the equator at noon... but walls count too, so that's kind of a wash. The reflectivity of the roof and walls help. So let's say all in all, 50% gets through.
This is why a car with 48 square feet (4.5 m²) of cabin gets so hot so fast - it's intaking 2400 watts/8000 BTU/hr of heat. That's the heating power of an oven. This is why leaving a baby in a car is so serious. I'm not equating a house to a car, just pointing out that solar gain is not to be armwaved or ignored, and is definitely your #1 load by a wide margin, maybe even by an order of magnitude.
So, your 1000sf (93 m²) house, is absorbing 50,000 watts of heat, or about 150,000 BTU/hr, from solar gain.
Yes, this is a stupidly large amount of energy. Too bad we can't harness it for something, eh? :)
Insulation slows it down. But only slows it. Given enough time, it still gets through.
Thermal "mass" works both for and against you. The thermal mass inside the insulation envelope helps your house resist changes in temperature. However the thermal mass outside the insulation envelope (roof, joists etc.) has been warmed by the sun all day, has reached 120-140F (50–60°C), and holds that heat for a few hours after sunset. That means its high heat is still trying to push through the insulation even after the sun has gone down. Conversely in the morning, even with full sun, you get some relief before the roof and joists heat up.
At least it's more efficient...
Your A/C unit interchanges with air, and in your case, you are saying the ultimate heat sink is cooler than the thing it's cooling. That gives a heat pump an advantage; it is pushing heat "downhill". You can check it with a load monitor (if the cycle length doesn't make it obvious), but you probably aren't using near as much power as on a hot-air day.
It's even better, of course, to interchange heat with groundwater. But that is a more specialty unit.