Na página de manual do socket (7) :
SO_RCVBUF Sets or gets the maximum socket receive buffer in bytes. The kernel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(2), and this doubled value is returned by getsockopt(2). The default value is set by the /proc/sys/net/core/rmem_default file, and the maximum allowed value is set by the /proc/sys/net/core/rmem_max file. The minimum (doubled) value for this option is 256.
Note que a página man indica que o kernel dobrará o valor do buffer especificado para a sobrecarga da escrituração contábil.