No, YOUR not making it up but someone else did
One of the reasons we(humans) have survived so long is our imagination, it helps us "reason" thru problems and come up with new, and sometimes better, ways to do things
Upside of imagination is that things don't have to "make sense" in the real world, you can approach any problem from a different point of view not hindered by reality.
And our imaginations can play tricks on us, we can firmly believe what we imagined, is REAL.
It becomes a memory, just like any other REAL memory, just one of the downsides of our imaginations.
To the person it is totally REAL, they are not lying, to them it happened just the way they "remembered" it
Dreams do that, almost anyone can remember a dream that they thought was REAL just after they woke up, but then they find out it wasn't, "darn, didn't win the lottery", lol, but for the moment it was a REAL memory