Command line -dpaper-size overrides ly:set-option 'paper-size

[Phil Burfitt]

If I have #(ly:set-option 'paper-size "letter") in a .ly file and compile 
with command line option -dpaper-size = "a4", the a4 setting overrides the 
letter setting and creates an a4 pdf!

Should and why is that happening?

Additionally, ly:get-option 'paper-size returns "letter" in the above 
situation, and returns "a4" if I remove the #(ly:set-option 'paper-size 
"letter") line from the file!

Confused,
Phil.