Because when I actually spoke to them, they deemed me the primary carer.
You gave them inaccurate information.
The calculator is simply a rough guide. It all depends on full disclosure of accurate information.
You were advised repeatedly that your ex allowing you to claim CB for both children, despite being 50/50, was the only reason the calculator and CMS deemed you eligible for maintenance. You were advised repeatedly that if he claimed CB he would win and that would stop your CMS claim.
You are not the primary parent. You are 50/50. And actually recently if you were to be pedantic he is the primary parent based on time. And time is what matters for CMs. Not doctors appointments and paperwork.
That he is still willing to be amicable, even now, says a lot for him. You'd be very wise to at least take some steps to protect your co-parenting relationship as this current determinedly destructive mindset you have cannot be positive for your children