[Exim] lookup mysql and {yes}{no}

hello,
according to the docs :
${if <condition> {yes}{no} }
will return 'yes' if the condition is true and 'no' if the condition fails.

why does this work the opposite way when i do a mysql lookup:
${if <mysql lookup> {yes}{no} } ?

I have to use:
${if <mysql lookup> {no}{yes} } so that when the SELECT query returns
non-null if matches 'yes'.

some clues welmcome :-)

thanks

julien.