Not really that weird when you think about it. We want to think of sign language as being a one to one relationship with whatever the actual spoken language is, but deaf people don't understand the spoken language at all. It's a completely different and separate thing. No surprise it tends to drop verbal conventions that are unnecessary to get across the meaning while developing its own.

The funny thing is French and ASL sign languages are basically the same. While the British sign language is completely different.

Anyway I had an epiphany years ago that ASL was worthless for me to learn unless I somehow have a deaf family member. Besides the deaf community being filled with unpleasant cunts bitter about people who can hear. If by whatever reason I need to communicate with one I can assume they are literate so we can just use the basic shared communication method we already have, writing.