Sengem
These people are supposedly dead but are standing in front of me in a body and able to use weapons and magic as if they were never dead to begin with.
1) Stuff on the Astral doesn't need to have an independent existence. People create items, golems, and places in the Astral all the time. Just because someone "dead" appears in Astral, it doesn't even mean they are there, but just something that looks and acts like them while you are watching.
2) As Milendil said, maybe they weren't you. If they were real lives, perhaps you just tapped into that memory-set. They could also have never really existed anywhere outside your mind, and being in the Astral you just projected that externally.
3) If they are past lives
they aren't you. A lot of theories of reincarnation (and incarnation actually) draw a distinction between what is currently making your meatbag kick, and what that force comes from. Past lives aren't stored in the Lower Self, but the Higher Self. Just as your computer doesn't store Gaiaonline, the internet does (well the Servers). So just because Sengem, the little slice of whatever your Soul is, is in a body, doesn't mean the last 10 animating sparks (Lower Self, Spirit, whatever) are in the body, but could be brought out of the Higher Self.
4) Even if you take a Non-Dualistic model of the Soul, and all your past life stuff is stored down in you, there is no reason why it can't manifest. In therapy people are taught to have conversations with their childhood self. If one is Astrally Adept, they can Project that childhood self outside of them as if it was an external reality, but it isn't. Just like when you look in a mirror, it looks like there is another you a few feet away, there isn't, it's just a reflection. That's all the Astral visions are in most cases, reflections.