When hiding from the Borg cube in "The Best of Both Worlds", the Enterprise is able to scan from within the nebula what's outside of it, whereas the Borg vessel is unable to scan what's inside.
Has it ever been explained (or is it deducible based on information presented in other episodes), whether this is because of the relative positions of the ships or due to distinct differences in Federation and Borg technology?
Or is there a third option I'm not considering (except for 'plot armor')?