Here is what I have found... The D17 amount is for 2017 401K, but the amount was actually deducted in 2018. Basically, taken out of 2018 Income, for 2017 401K contribution.
So, the 2017 401K contribution was less by this amount reflected under D17. Since, it was the employers mistake, they took that amount from the 2018 pay to be accounted for the 2017 401K contribution. They have then reflected this in the W2 as D17.
