Termination of high frequency PCB traces


I recently started to design PCB card dealing with ADC and HF signals, which requires to take into account impedance match of lines and their devices ports in/out.

In my studies I've learnt transmissions lines, and matching impedance is (logically) happening at both sides, so I cannot understand why I always see non-symmetrical line terminations in suggested schematics to terminate different interfaces such as : 

Why terminations are always happening on one side only ? Why shouldn't I put thevenin termination on both sides for instance ?

I guess it is related to internal impedance of in/outs but not sure...

Thanks for your enlightenment !