That's not how it works with BMI, they just fix a price to every airport, and add it up.
For instance ZRH cost more than FRA, although the truth should be the opposite.
If you make lots of stops = lots of airports, you could get well over $1000 tax.
I've seen once an excel sheet, where someone added all airports and prices for bmi...
To amswer you: if bmi forgot to attribute a value, or decided to go cheap on the US hubs, you are good.