Why does key = value work in some cases, but not others?

config_block_explanation.rb

c = Confstruct::Configuration.new(:foo => {})
c.configure do |c1|
  # the block expects one param, so we've remained bound to
  # the outer scope and yielded c.
  # IOW, self == whatever self was outside the block, and c1 == c.

  c1.x 123         # calls c1.method_missing(:x, 123)
  c1.y = 456       # calls c1.method_missing(:y=, 123)
  c1.foo.bar = 789 # calls ca.foo.method_missing(:bar=, 789) 
end
=> {:x=>123, :y=>456, :foo=>{:bar=>789}}

c = Confstruct::Configuration.new(:foo => {})
c.configure do
  # the block expects no params, so we're in c's instance
  # scope right now. IOW, self == c.

  x 123         # calls self.method_missing(:n, 123)
  y = 456       # local assignments take precedence, so since c has
                # no method called #y=, this just assigns
                # the value 456 to a block-local variable called y  
  foo.bar = 789 # calls self.foo.method_missing(:bar, 789)
end
=> {:x=>123, :foo=>{:bar=>789}}