Thing is, it's more than just forgotten memories.  If you look at Serack's OP, you'll find that Mort had moved away from his California-style house by the beginning of DB.  So either Mort moved back so the place he had previously sold - just in time for Harry to go there (remember he writes down a specific address, not just "take me to Morty"), or *something else* is going on.  

My current take on things is that's it's an "Enterprise C" sort of scenario, i.e. Harry has been transported to a sort of alternate universe where "history" is different:

effects so far:

1) Mort never moved.
2) He's known about his grandfather/daughter for a while.
3) His hand never got burned.

Following this logic down a rabbit hole, all this suggests that the point of change took place between chapter 10 of Dead Beat (when he visits Mort) and the story of Death Masks (when Maggie is conceived).

So why?

- One possibility is that the "timeline shift" is actually an attempt by Uriel et al. to help Harry (i.e. make it easier for him to solve his murder and thereby get back).

- The other possibility I am currently playing with is that the murder weapon itself was a sort of Entropy Curse (possibly pointing toward Lord Raith as the culprit - which matches the timeline):

Consider that a turkey falling out of an airplane flying at 30,000 feet can take around a minute to hit the ground.  This means that in order for a falling turkey hit a moving target (especially when the target gets changed at the last moment), you need to somehow know in advance where your target will be standing.  Assuming we accept this, it means that an entropy curse can somehow reach back in time to achieve the currently desired effects.  Yadda Yadda.

Wait (if this has been brought up, I apologize - I haven't gotten to the end of the thread before posting):

What if the discrepancies are because Harry died before he was able to time travel in a later book.  As in, somewhere in the future, he would be going back and meddling in various ways (or he would send someone else back).  Therefore, he remembers certain things, but not others - and changes he would have gone back and done have been undone...

Ok - that all hurts.  The paradox, it burns...

[attempt to edit for clarity - capitalizing Changes in the second to last sentence was a cute play on book names, but really confusing.]