Hello, Orozconleche,
I have no scientific reasoning or proof, but just from personal experience, it seems to me that heated foods seem to render the heat of chiles more-so than cold foods do. Or makes them more interactive to our mouth's capsaicin/pain receptors? It has seemed to me, imho, that the same sauce with the same amount of heat, tends to taste hotter when the sauce is hot as opposed to when the sauce is cold.
Might be something as simple as when the pico is first made it's usually eaten a room temperature, and the day later, it's refrigerated. I also have noted that with fresh pico, eaten within a day or two, it seems to be milder the next day. It might also be due to some interaction of the natural sugars present in tomatoes that off-sets the heat of the chiles.
But then again, the hot versions our fresh salsa, sold in local stores, gets hotter after the first few days. This is after 3-5 days that the salsa gets hotter, not just the next day.
Sorry, Oroz~, not much of a help here~~~ just sharing some personal observations.