When it comes to entering the US as a non-LPR/citizen, you’d do well to understand that it is fully down to the discretion of CBP.
I had a similar experience to your fiancée once: entering for comfortably less than permitted (but longer than most people could practically do), letter from employer, savings, mortgage statements showing ties to home country etc etc.
I was going by the book and had evidence to prove it. I was still *this* close to not being let in. The officer was just super skeptical, rightly or wrongly.
You just have to understand that they can give a hard time or deny entry and rather than getting angry that it isn’t fair, which I have sympathy with, find a way of avoiding the situation in the future - ie use the B2 less.