It was strange until I realised what was going on...

I maximised iTunes and pressed play, Nine Inch Nails doing a cover of Joy Division's 'Dead Souls'...sweet. Pressed the shuffle button because I didn't know what I wanted to listen to.....Another NIN song, and then another. I started to think it was odd to have three in a row when a fourth came on. Hmmmm, oh, i see, i'd done a search for NIN in my playlist so it was only shuffling the results of the search. DUH!