Why did 'should' become the going nomenclature of BDD framworks?

'Should' connotes 'ought', as if something ought to do xyz, but
doesn't necessarily have to. I don't know about you, but I don't write
my tests/specs that way!

'Must' is the word that denotes what our tests/specs do.

  my_method.must.do_the_right_thing

not

  my_method.should.do_the_right_thing

'Must' is a shorter word too. So why didn't it become the word of
choice here?

I think long dead Grammar teachers are rolling in their graves.


-7. Muse.