Pioneer DJM400K master output problem.

Hi. I'm trying to record my sets with SL3-serato and my left channel is about 0.3-0.5 db louder than the right channel. I tried different rca cables and the internal sound card from my mac and had the same results. I know it's not a big issue, i had this problems in the past with other mixers as well (not pioneer..) but i want to know why is that happening??? Anyone knows?

Does my mixer needs repair?

Thank you.