Thread from when this issue first arose in October:
NHL and NHLPA having problems
I will guess that the largest chunk of the $20M adjustment in HRR came from including a portion the $25M CoG subsidy in the Final HRR numbers - as was speculated/predicted in that thread.
Note that this discrepancy will not effect this seasons cap - a discrepancy between the accepted Initial HRR Report (which the cap is based on) and Final HRR Report would only be reflected in the cap if it were large enough to change the calculated Midpoint by >$3M. This change (if accounted for earlier) would have only bumped the Midpoint (and thus the Upper Limit) by ~$380K - $20M * 57% / 30 teams.