This seems to work the other way too. Lot's of hot countries seem to set the AC to 18c. I did trip to south america, and I needed warm clothing specifically for air conditioned busses and dormitories.

In the UK (where I live) I believe the recommended guideline is 21-24c

Yes, I also noticed it in SE Asia. I guess, due to the fact that AC was (and in some places is) too expensive for the majority of population, cold air feels like a luxury attribute.

American here, from the South. I've noticed the same sort of phenomenon travelling to the Canadian plains in winter, or among Northern transplants to the South in midsummer. Canadian sitting rooms felt like greenhouses to me (80+ Fahrenheit) despite it being 20 below outside, and my boss has commented before on visiting Southern households where the A/C is set to 68 in July.

My theory is that it's more about not-to-exceed ceilings [or the converse, floor] temperatures rather than average temperatures. When you have the huge temperature differential between the conditioned space and the outside, opening a door or something can affect the conditioned space's mean temperature by a huge amount for a short time. By over-conditioning or over-heating a space, you're more likely to be able to ensure the interior temperature never rises over ~75F, than if you'd set it at a comfortable 72F or so. Or in winter, it won't drop below a minimum comfortable temperature indoors when someone has to go outdoors.

Of course, once you've adapted to that habit, you may not always make seasonable adjustments to those settings.

The coldest parts usually have double doors at the entrances right? So temperature doesn't really swing so much from people transit. Beside, office buildings are usually extremely large compared to volume of exchanged air at the entrance.

As a native Canadian, I haven't experienced this so it's a somewhat surprising data point to me. Although perhaps I'm just used to it :)

I had the opporsite experience at a conference in Australia. I ended up bringing a jacket to wear once we got inside.