Threshold voltage dependence on Vdb

[opcode0x90]

Why does the Vth of MOSFETs depends on Vsb and not Vdb even though MOSFETs are symmetrical devices? Is it a convention to choose Vsb over Vdb or am I missing out on something more fundamental?

 

[Kholdstare]

Because the source is usually kept at the constant. Also after pinch-off the drain is separated from channel charge by a depletion layer.