Recovery Rebate Credit - incorrect amount calculated?

Hello,

I didn't get the first round of stimulus payments. I was supposed to get $2400 (married filing jointly for 2020) plus $500 for my son.

In the second round I got $1,200. I was supposed to get $1200 (married filing jointly) plus $600 for my son. My wife has an ITIN, not a SSN. 

In the TurboTax Software I was asked if I got any of the checks. I put $0 for the first payment and $1200 for the second one. I was expecting at least $2900 (corresponding to the first check) and not sure if we would get additionally $600 for the second as my wife does not have a SSN, however, the software calculated $1700. Could you please let me know how it is actually calculated? Thank you so much.