SSDI 1099 Lump Sum Backpay

Q1.  I received my SSA 1099 for my lump sum backpay  (2018, 2019, 2020).  However, I had not completed my tax return for 2020 at the time.  Does that make a difference in the calculation of the SSDI lump sum backpay?  *Note:  I applied for SSDI in 2019 and was approved by a judge in 2021. Back pay started May 29, 2018 thru 

 

Q2.  The amount shown on the SSDI 1099; box 3- paid by check or direct deposit - is NOT the amount I received. Why is the amount I received $69,144.00 LESS than what I received?  And, why do the numbers in the description at bottom of box 3 not add up to the total amount I received?  I've tried talking to an SSA Rep. but they did not understand why the calculations don't add up?  I can't file my 2021 tax return because of incorrect numbers calculated.   What do I do?