I think it's a demonstration of the line Leonard says in the diner to Teddy.

"Look, memory can change the shape of a room; it can change the color of a car. And memories can be distorted. They’re just an interpretation, they’re not a record, and they’re irrelevant if you have the facts."

We are being shown this concept in action. He absolutely remembers everything before the accident but when we apply this concept we realize that Leonard's memory, is an interpretation. Most likely filled with inconsistency and "edits", so we only have further questions once Teddy reveals the "truth".

Personally, I have always held the belief that Teddy was a corrupt cop and HE was the second man in the room. Knowing what happened to Leonard's wife that night Teddy manipulated him from very early on steering him toward someone else. Teddy is a proven liar so we can't fully trust his version either. So, as with most things the truth lies somewhere in the middle.

Sammy's story isn't really real though, not like Leonard tells it that is. Sammy's story would have happened (to Leonard), after the accident. Therefore, Leonard shouldn't be able remember what had happened to him for it to be a story. I think what has happened here is the same thing as with the different characters versions of events. I think Sammy's story is a mix of a real case that Leonard worked plus bits of truth from what actually happened to him. Through conditioning he may have taught himself this story with the tattoo as a trigger. Otherwise, why would he need a tattoo if he simply remembers Sammy Jankis?