Spelunky has gained the reputation of being a game you study and master as much as play. To become good, and the level of skill needed to really make your mark on the community is incredibly high, there are multiple systems you have to both understand and then manipulate reflexively. You can never rely on knowing the level, because the levels are different every time you play.
The video above goes into some detail about how this happens — although you should certainly read Spelunky by Derek Yu, the game's creator, if you'd like to learn more or would like to learn more about game design — and it should give you a bit more appreciation for how the levels are created and why they feel so satisfying to play every time you pick up the game.
Which is why I put the word "perfect" in quotes in the headline for this story. You may get better seeds in some rounds than others, and you can read this article for a good explanation of what a seed is and why it matters, but each level feels like it was guided by a designer. You'll never encounter a level that's uninteresting, or unplayable. I'd put a random map of Spelunky up against a purely authored map of almost any other 2D game, and I'd be confident the Spelunky map would feel, to put it rather broadly, more delightful.
You can even try your hand at this process a bit using a tool from 2013. The video above does a great job of explaining why Spelunky is so special, along with a bit of the technology and design that made it all possible.
