Yes, the meaning of the = is different in patterns and in the inline use.
In the inline use the = actually has the syntax pattern = expression where the expression on the RHS is first evaluated and then the value of the expression is matched against the pattern. So it has a very strictly defined right-to-left semantics.
However, in a pattern it is an alias where both sides are pattern matched and both matches must succeed. All the variables in both patterns are bound if the matches succeed. It is commonly used for the case where you want to match and extract parts of a structure and have a reference to the whole. Like in you examples with maps and structs. It lets you have your cake and it.
Using the same operator perhaps wasn’t the smartest thing but it was inherited from Erlang so you can blame them (me). 
