Are pattern variables additive?

[David Caldwell]

A simple test case:

x="hello"

%.b: x+="a"

%.a.b: x+="a+b"

david.a.b: x+="c"

david.a.b:
        @echo $(x)

$make -f pattern-vars.make david.a.b
hello a c
$make -v
GNU Make 3.80

I would have expected the output to be "hello a a+b c" or perhaps "hello 
a+b a c".

Is this a bug?

-David