The Hand is well-known for using resurrection to maintain both their own immortality and that of others. As we see on Iron Fist, it can lead to the person it is used on becoming more and more violent and disconnected from their previous self, but they still seem to maintain their general memories and personality. The leaders of the Hand have been using this process for millennia.
However, in The Defenders, both the leaders of the Hand and
Stick believe that Elektra is gone, and that she is an entirely new person as the Black Sky.
They later turn out to be rather mistaken: Elektra seems to recall most of her previous life, as indicated by her conversations with Daredevil, though it’s not clear that this was initially the case. Still, it’s not a surprise that she had some initial disorientation: we saw that in Iron Fist too.
Why would the Hand believe that her memories are gone due to resurrection, when the process doesn’t seem to work that way for anyone else?