"The client did have taxable Social Security Benefits but that is already in the equation to determine Adjusted Gross Income."

It's not the Amount. It's a payment mechanism. It's one of the ways they judge a taxpayer is entitled to EIP funding, and it can be a duplicate payment to the clients that are both SS recipients and tax filing entities. We've seen this posted here, already.

"Also, the IRS website states how the 3rd payment is to be calculated and based on their document, the client received all he was due with the EFT payment that was received in March. It appears the IRS is making mistakes reconciling these payments."

Yes; that's happening with EIP 1, 2 and 3.

"because his adjusted income in 2019 was higher. He only received $887.80."

Based on the Projection of what 2020 would be when filed, and projecting no changes from 2019, if it was already filed. Everything paid out was paid on Projection, and they all are based on whatever is the most recent tax info on file, even if, from 2018 to 2020, you had triplets and lost a spouse. Factually, everything is settled on the tax return, later.

"In the IRS calculations they are saying the Recovery rebate credit should be $801.65 not $312.00."

The IRS moves in mysterious ways.