This is sometimes used by financial institutions that are non-market makers to report larger transactions that actually occurred during market hours, but since they do not have access to the ACT (Automated Confirmation Transaction Service) use Form T to report. In essence, bypassing the MMs.
MMs are basically prohibited from these "Off-Market transactions".
A pattern or practice of late reporting without exceptional circumstances may be considered conduct inconsistent with high standards of commercial honor and just and equitable principles of trade, in violation of Rule 2110.
These ”Off Market” trades are typically used by larger investors to buy larger lots at prearranged prices without risking to drive the price upward or downward.