Signs have the following definable components:

• Handshape – the shape of the hand(s)
• Palm Orientation – palm up, down, in, out, or sideways makes a difference
• Location – forehead, chin, chest, forward, back…all these give various meanings to the same apparent hand-waving
• Movement – single, double, direction, speed, size…you get the idea…

The one undeclared component that makes a huge difference is Context…  The sign I learned as METAL is used by some folks up here to mean TRASH.  LETTUCE and TRASH seem to be the same (trash salad, anyone?), and the sign I learned as PURSE is commonly used to reference TRASH.

Another aspect that keeps life interesting, just as in English, is dialect and language shift, which causes signs to differ across regions, generations, and genders.  This would be why a sentence about putting lettuce in a metal purse could be misunderstood as very trashy.