Memories, In Rocky Road Elsa stated she does not remember who put her in the urn or why; However; Elsa seeks her sister at any cost. She wants Anna, and nothing else matters. She just wants her sister back as she told Emma in White Out.
Now, bring in the Memories, what happens in the modern day only get Elsa so close to discovering the truth of what happened to her and why it occured. My path of thinking is that the Arendelle of the past scenes that occur, are memories, in this case, Elsa's genetic memories from a few days to maybe a few months before ending up in the urn itself.
Now, Elsa cannot recall who or why or how she ended up in the urn; which means that particular memory... block, lets call it that, is locked, she can't access it, if she tries it only gives back a fog she does not want to see {a very bad headache occurs}. Therefore, the past events at Arendelle occur a few weeks before ? {I'm going to assume that Elsa remembers everything UP TO 12 hours before 'waking up' inside the urn}
Now; As she 'relives' the memories leading up to the point of her getting stuck in the urn and why, she is close in modern day world; to discovering the truth of her mothers journal and what she really is as a whole. And the closer she gets to discovering the truth, the stronger {I personally hope} she gets with her powers.
These two 'stories' dovetail it seems into one intiwned web of both deception and betrayal; both in the present and in the past. The closer Elsa gets to discovering the truth, the deeper and more complicated the deception and betrayal become, both in the past 'memories' and the present day world.