How important is transmutation circle pattern?
Note that Al's alchemy tends to always revolve around the displacement of material. Comparatively, Mustang's alchemy revolves around chemistry (igniting materials) 

Al may be using the same circle which allows him to use a form of "telepathy" (for lack of a better word) to piece things together. 

From many instances of alchemy being used, you can see that the intent of the alchemist changes the effect of the transmutation.


Mustang's glove carries the same symbol, but Mustang has many fire transmutations up his sleeve.
Armstrong's glove is the same (for earth and metal related transmutations) 


This is a recurring pattern for many alchemists. Their circle limits their alchemy to a specific realm (fire, earth, destruction, displacement, ...) but they use the same circle to perform many different transmutations within that realm.

Alchemy seems to be a combination of the circle (method of transmutation => Al's circle displaces material) and the alchemist's intent (how to apply that method => Al puzzles the thing together again).