Question Regarding Signing

I have been learning ASL for years now and am pretty good at it, but obviously have a lot to learn. Someone sent me a video of them signing today (Deaf) and I really struggled to understand because of how she signs. We aren’t close enough that I wanted to ask them this question, so I brought it here:

Why when you become more fluent do signs change? Is this common for Deaf people?

She signs VERY fast and it almost seems like the signs are incomplete. For example, she signed “sister” but used one hand and basically brought the hand from chin down, no other hand used. I know how to sign “sister” so I figured it out eventually, but it took a while.

Is it almost like slang where people fluent start signing “sloppy”? I don’t want to use that word because it’s not actually sloppy, it’s just less structured than I’m used to. I feel like I see a sign signed differently than I’m used to and I completely blank.

Can anyone explain this to me? Is it common for all Deaf people, or just some? Thanks!