Why did Jonas misinterpret the Receiver's job of transmitting memories from the past?

I think that you are probably asking about the part in
Chapter 10 when Jonas first goes to start training for his new "job" as Receiver of
Memories.  If that's what you are asking about, the best answer I have is that Jonas
cannot (at that point) understand the idea that the Giver could carry the memories of
all these times past that Jonas doesn't even know about.

In
the community where Jonas lives, everything has been the same pretty much forever (as
far as anyone but the Receiver knows).  So Jonas doesn't think of the fact that there
could have been other times and other places when things were different.  If everything
has always been the same, what would be the point of having these memories -- they would
all just be the same as things are now (except for little personal memories, which is
what Jonas expects).

So the sameness that Jonas has grown
up in makes him think that the way things are here and now are how they have been always
and everywhere.  Therefore, he can't anticipate what kinds of memories the Giver really
has.