Obscure, Christine McVie-heavy re-release
I had to dig to find anything out on this, but according to www.systemrecords.co.uk, this is "early Fleetwood Mac from 1970, plus 4 tracks from 1969 by the pre-Mac Christine Perfect Band. Mixture of studio and live tracks concentrating on the post-Peter Green Fleetwood Mac with Jeremy Spencer and Danny Kirwan."
So the "different sound" here is due to the fact that this is one of the many phases of the band that predates the one most of us are familiar with. Look them up on wikipedia--quite an interesting read on a very dynamic band.