Format strings round differently than Math functions

[Mike]

can anyone explain to me why, given:

decimal dec = 84.50M;

Convert.ToInt32(dec).ToString()
and
Math.Round(dec,0).ToString()

both correctly output "84"

while

dec.ToString("N0") outputs "85" !?

I'm quite content that the math and convert functions round to even,
i.e. "banker's rounding" as the docs say, BUT WHY, WHY would format
strings round DIFFERENTLY?

Thanks,
Mike