Clock vs Boud Rate

Boud Rate(BR)
Divisor (DIV)
Clock (CLK)

CLK = BR * 16 * DIV

why?

http://www.intel.com/design/mcs96/manuals/272238/
http://www.intel.com/design/mcs96/technote/2673.htm
http://www.eel.tsint.edu.tw/teacher/hykuo/arm_exp/EX5_UART.pdf
http://ftp.icpdas.com/pub/cd/iocard/pci/napdos/multiport/manual/vxc_customized_baudrate_chinese.pdf
http://www.pjrc.com/tech/8051/autobaud.html