My tone is an acquired taste. 😉 It's intended to first get your attention. I see that purpose was accomplished. We intelligent, educated people have a tendency to make similar mistakes. I've seen it for the 19 years, I've been a member here and I've made them myself. One of the most common mistakes is to overthink things. The keys are to first study instructions and the questions on the forms very carefully. While doing so, it is critical to interpret literally, and answer accurately. Little things like commas, if, and, or etc. are often missed. The highly educated lawyers that approve the language in forms and instructions, use a lot of those details of the language. If you miss them, you'll make mistakes.
They don't just say enter the total income, because if you file a 1040EZ, there is no total income line to use. So, the comma you missed when incorrectly concluding you were to enter the AGI number was very important. Ironically, AGI and Total income are the same number, quite often, maybe even most of the time, but not always.
The I-864 is the most complicated form in the process. Be sure to carefully read the questions and instructions on the face of the forms too.