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)

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I guess I'm talking to myself here...

I discovered that by modifying the line
        target2 : foo +=
to
        target2 : foo += ''
I get the right results.  I guess it's a side-effect of how
make parses
a variable when it's part of a rule.

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