As far as I can understand:
put you have the keys:
A
AB
ABC
and you try to locate 'AA'
lopPartialKey: no record found, since none of the above keys is starting with 'AA'
lopFindNearest: the nearest is 'AB'

put you have the keys:
ADDICT
ASSERT
AZZURK
and you try to locate 'ASS'
lopPartialKye 'ASSERT' is returned, since 'ASS' is a part of 'ASSERT'
lopFindNearest: the nearest is 'ASSERT'

hope I'm right ;)
regards
Marco Menardi