immediate/defered variables

[Der Herr Hofrat]

Hi !

 trying to understand the concept of immediate and defered variables I wrote
 up a simple makefile - but it todes not produce what I expected

--- Makefile --- 
files_a := *.c
files_b = *.c

.PHONY: show
show:
        touch junk.c
        @echo ${files_a}
        @echo ${files_b}
        rm -f junk.c
        @echo ${files_a}
        @echo ${files_b}
------

the directory contains hello.c so my expectation was to see

touch junk.c
hello.c
hello.c junk.c
rm -f junk.c
hello.c
hello.c 

but I actually get:
 
touch junk.c
hello.c junk.c
hello.c junk.c
rm -f junk.c
hello.c
hello.c

So the immediate variable files_a was NOT expanded immediatly
but was passed to echo as *.c .

I assume my concept of immediate vs. defered is wrong - so could anyone explain
me how to read the mkae manual corectly ??

thx !
hofrat