OMAP-L137: Trace Impedance and Source Terminating Resistance Question for OMAP-L137

Part Number:OMAP-L137

In the starter kit design schematics, they use 22 ohms source terminating resistor packs to connect the DSP to the SDRAM. However, in the gerber files, they specify that the trace impedance should be 50 ohms for the corresponding traces. 

Can you please explain why they decided to go from DSP -> 22 ohm resistor -> 50 ohm impedance trace -> SDRAM
instead of
DSP-> 50 ohm resistor -> 50 ohm impedance trace -> SDRAM
or
DSP -> 22 ohm resistor -> 2 2 ohm impedance trace -> SDRAM

Here's the link to the file:

http://support.spectrumdigital.com/boards/evmomapl137/revd/

Thanks