Memories in a Bottle

[Matthew Buffington]

No: HPFGUIDX 141210

When going through posts about Harry possibly talking with a portrait 
DD in book 7, I had an idea.
In GoF and OotP, DD always retrieves his own memories by putting his 
wand to his head, pulling the memory out, and then putting the memory 
in the pensive. However in HBP, DD retrieves the memories from 
bottles. Of course, this makes sense when the memory belongs to 
someone else. But the memory where LV asks DD again to work at 
Hogwarts, DD retrieves his own memory from a bottle and not from his 
own head. Why?
Is it possible that DD might have stored up some of his memories at 
Hogwarts so that Harry could return and check them out for himself? 
Where could these memories be stored at? Could we then find out the 
real reason that DD trusted Snape so much?
I would appreciate any thoughts.

Matt