Couldn't Misa have told Kira L's life span?
L was killed by a Death Note (Rem's). When someone is killed by a Shinigami's Death Note, the remaining life span is added to the Shinigami's life. This means the life span seen using the Shinigami eyes doesn't account for deaths caused by the Death Note (since that difference is added to their life). The important part of what the Shinigami eyes does for Misa is that she can see L's real name.