${(z)} parsing of multiple array assignments

[Daniel Shahaf <d.s@xxxxxxxxxxxxxxxxxx>]

In the following two cases, why are the assignments to $b
parsed differently to the assignments to $a?

    % pz() { print -rl -- ${(qqqq)${(z)1}} }

    % pz 'a=() b=()' 
    $'a=('
    $')'
    $'b='
    $'()'

    % pz 'a=(foo) b=(bar)' 
    $'a=('
    $'foo'
    $')'
    $'b=(bar)'

Thanks,

Daniel