The word is "retransmission" . . .
For the DS (relative to the end user) the transmitter is the DSLAM/MSAN, the xTU-C.
For the US (relative to the end user) the transmitter is the modem(/router), the xTU-R.
To be able to retransmit a data sequence, the device needs a suitably sized buffer to hold that data. It is far easier to implement at the DSLAM/MSAN than it is at a modem(/router).
Regard it as a hardware limitation.