Why the conversion-m240

When I sync 50 songs to my m240 using WMP, some of them will convert from mp3 to wma. What is going on here.  Every file on my computer is in mp3 format, so why the automatic conversion for some songs only?

where does the songs came from? if you ripped songs from CD the default settings of WMP is WMA. And as far as i know if you sync songs on the device it will only transfer the songs and it will not convert it to another format. BTW i think it doesnt matter if you transfer WMA or MP3 songs on the device because the device can read the said format.