IRS and your state taxing authorities are not always right in how they calculate things. For one, they tend to make all assumptions completely in their favor, completely disregarding the very high probabilities that at least some of that gray area belongs to you! Second, IRS agents make mistakes too. It’s always good to double-check calculations, even if it’s the IRS’ numbers.
It’s also very good to know how to fix these kinds of problems if they happen to you. Knowing IRS structure, the rules and regulations, and the internal processes available to you to address this situation is my fortè.