Only the seeds of the habaneros I bought are hot – low quality, or simply the nature of chili

I'm a newbie when it comes to any type of chili. I'm alright at eating food with chili, but don't really have any experience with using it.

I bought the one I used at a local supermarket. I first tried eating a small slice to measure its hotness, but couldn't really feel much heat at all. Pleasant flavor though.

Then I tried to eat a small slice with one (1) seed on it, and this time, it was pretty damn strong, I'll admit. But the "flesh" itself was not strong. The food I made (ground beef, crushed tomatoes, onion) was about ~1.5kg total and filled my pan, but when I added half the chopped habanero with ~6 seeds there was not noticeable hotness at all. I didn't notice this until I served since my tastebuds were giving me false readings from the first tasting I did.

So, this lack of hotness, is it because

1. That's how habaneros work (surprised me that I didn't get any hotness though)
2. The type I bought was not very strong.
3. I prepared it all wrong. Extract flavor using oil first?
4. My supermarket sells non-fresh / low quality habaneros?

Or something else?