target-specific variable behavior

could someone please explain as to why this is the behavior of
target-specific variable?
the output that I'm expecting is

[expected output]
target1, bar1 bar2
target2, bar1

but this is what i got instead

[real output]
target1, bar1 bar2
target2,


[makefile]
foo := bar1

all : target1 target2

target1 : foo += bar2
target1 :
	@echo target1, $(foo)

target2 : foo += 
target2 :
	@echo target2, $(foo)