Because more than anyone, Spurs fans know that bragging rights are incredibly vulnerable and can be lost even when you are clearly the superior and more successful side. Consider how much they mocked our attempts to win the Champions League at a time when they couldn't even qualify for the competition.
When we're bellowing 'Wem-ber-ley' at them on Sunday and possibly end their title hopes by taking points off them, it will not feel to them like the page has turned on a glorious new era of north London domination.