Reading between the lines = Making stuff up
They are close, sometimes readers make stuff up with their interpretations, but there are also times in which the writer put the message between the lines on purpose, specially if they want to get through censorship.
It is something we studied in primary school in Spain, and saw examples of many recent writers, some of them are still alive. Oh, in case you don't know, Spain was a dictatorship until 1975, thus why many writers and artists that disagreed with Franco and his rule had to use these type of methods to get the message they wanted across.
In other words, from the reader point of view it is the same process, interpretation. The big difference IMO is that you can read between lines when the writer actually had the intention for it. So I would avoid refering to it in such a derogatory way, seeing as it is a phenomenon that happens and is studied.