post below explains why this is written but writing and then writing SBUB does not means reading the same value out, when you write to the SBUF you write to a address multiplexed Serial transmit buffer register, while when you read it comes from the serial receive buffer. their address is same but they are not the physically same registers.

Intel's Manual says:

"The serial Data Buffer is actually two sepatrate registers, A transmitt buffer and a receive buffer. When data is moved to sbuf it goes to the transmit where it is held fro serial tranmission (Moving A byte to SBUF what initiates the transmission.) If data is moved from the SBUF, Comes from the receive buffer."

Abhishek