:) you would expect it yes :) (I did)
Without the help of the guy's here I would have probably taken a while
to figure it out myself.
It's clear for me now. The new rule, written into my brain: methodes
whose names end in '=' always return the first argument. Writing this
down made me think: with multiple arguments, does it return them all,
or just the first. I gave it a try, and it seems like I'm not allowed
to give more then one argument to an methodes whose names end in '='.

Can anyone confirm this as a rule?

irb(main):001:0> class A
irb(main):002:1> def a=(b)
irb(main):003:2> puts 'in a=(b)'
irb(main):004:2> @a = b
irb(main):005:2> return 'c'
irb(main):006:2> end
irb(main):007:1> def a
irb(main):008:2> return a
irb(main):009:2> end
irb(main):010:1> end
=> nil
irb(main):011:0> aa = A.new
=> #<A:0x2c1067c>
irb(main):012:0> aa.a = 'b'
in a=(b)
=> "b"
irb(main):013:0> class X
irb(main):014:1> def x=(y, z)
irb(main):015:2> puts 'in x=(y, z)'
irb(main):016:2> @x = [y, z]
irb(main):017:2> return 'w'
irb(main):018:2> end
irb(main):019:1> def x
irb(main):020:2> return @x
irb(main):021:2> end
irb(main):022:1> end
=> nil
irb(main):023:0> xx = X.new
=> #<X:0x2bfd194>
irb(main):024:0> xx.x=('y', 'z')
SyntaxError: compile error
(irb):24: syntax error
xx.x=('y', 'z')
          ^
        from (irb):24
irb(main):025:0>