Total Disability based on Individual Unemployability pays the 100% rate ($3,938.58/mo) even when your combined rating is lower, if service-connected conditions keep you from holding substantially gainful work.
The schedular thresholds are set by
38 CFR 4.16. One condition rated 60% or more qualifies. So does a combined rating of 70% or more where at least one condition is rated 40% or more. Conditions from a single accident, a single body system, or paired limbs can be treated as one for that test, which is how many veterans reach the threshold they thought they had missed.
Missing the numbers does not end it. Under 4.16(b) a claim can still be granted on an extraschedular basis if the evidence shows you cannot hold substantially gainful work. "Substantially gainful" means earning above the federal poverty threshold in work that is not sheltered or specially accommodated. Working part time, or holding a job your employer has bent the rules to keep you in, does not disqualify you.
This check reads the ratings VA has actually assigned, never your targets. It is an estimate for planning, not a VA determination, and it cannot see anything outside the conditions you have entered.