ASL isn't a set of vocabulary governed by rules -- it is a caravan winding and
twisting as it carries Deaf people forward throughout the journey of our lives.
Sign choices are based on the flow of the caravan. If a significant number
of people start travelling down one path (start using a new sign or modifying an
old one) chances are more and more people will head down that same path. The
path becomes "right" by virtue of the fact that lots of people are now going
that direction.
If your goal is to meet and interact with others then the right path is the one
that other people are on.
You can either stand there alone looking at the map (your ASL book) and yelling
"You are going the wrong way! This other way is the way we have gone in the
past!" -- or you can abandon the old path, join the group, and enjoy the
journey.
Just because you learned something from your ASL teacher, an online group, or a
website -- doesn't make it right. What you learned might have been useful
in one class, city, state, or region -- for a while. Over time, what you learned
may become "less and less useful" until eventually might become confusing to
others with whom you are trying to communicate.
Language changes and evolves.
Notes:
Definition of caravan: A group of people, especially traders or pilgrims,
traveling together across a desert in Asia or North Africa.