Recovery Rebate Credit Calculation Seems Off

I filed my taxes through TurboTax. When the TurboTax system asked me about the stimulus payments I received, I noted that I received $140 for the first check and it showed that I was eligible for $460. I submitted my tax return not realizing the recovery rebate was line 30 on the 1040 tax return and thought something may come up  after the return was processed saying I was owed a credit on the stimulus payment and didn't know to look for an amount on the 1040. On the TurboTax FAQs it notes that for filers with income above those amounts, the credit amount is reduced by $5 for each $100 above the $75,000/$112,500/$150,000 thresholds and an individual without children will not receive any credit if their AGI exceeds $87,000. Shouldn't that threshold for an individual with no children to receive no credit be at $99,000 or if if it is $87,000 why did the software tell me I was eligible for $460? When I went through the Recovery Rebate Credit Worksheet available from the IRS website it looks like I should be eligible for an additional $320 (for a total of $460 that was shown on the computer when I went through the questions but the difference didn't show up on my 1040 and wasn't added to my tax return and I can't find any reference to the AGI threshold being $87,000 to be able to receive the credit and wondering if I can somehow amend my return to get the additional stimulus money I may be owed.