She is described as Mab’s second, commanding 5he Battle at the Gates so I wonder if she is currently more powerful than the Winter Lady?. Lea is not part of the Mantles so the balance of power shouldn’t have altered, and Mab wouldn’t have transferred part of her power to Lea.
Power can be transferred at Halloween, so did Mab call in a favour or favours to transfer power to Lea? If so, who gave up some of their power? Obviously someone or several someone’s in Winter.
Why do this? Partly for the Athame but I suspect partly to put Lea beyond Nemesis touch. This would make sense, both Ladies were Nemfected and Lea may have originally been at the same or slightly lower level, so that level is susceptible. Mab now has a second in command powerful enough to resist Nemesis who does not upset the balance between Summer and Winter.