Okay, an actual explanation....
Deep Purple has had many lineups over the years. Because of this, the media has unofficially dubbed each lineup using Marks. So the original lineup was Mark I, Mark II was the second lineup, etc. Think they're on Mark IX right now, but it's been the most stable, I believe since 2002 so almost going 20 years. But they're also a legacy act at this point, so there's that.
Generally speaking, the different Marks aren't so radically different sound wise, especially after Mark V. But Mark I is probably an interesting case. It is possibly the most different of them, it almost sounds like a different band completely. It was, of course, the original lineup. And it was the lineup that had the biggest hits in the states. Yet you ask your average rock fan, what they probably think is the original lineup is actually the Mk. II lineup, which is the lineup that in retrospect stood the test of time the best, and their most well known songs to modern audiences came from.
Basically, Hush, Kentucky Woman, River Deep Mountain High were Mk. I, and these were all relatively big hits at the time, but (aside from Hush) are largely forgotten nowadays.
Smoke on the Water, Space Truckin, Highway Star, Woman from Tokyo are all Mk. II, and I'm sure everybody here has heard at least one if not all these songs. It's their most famous to modern audiences, yet was not their original lineup or original sound.....and actually not as high charting as their original lineup in the US (but they were more popular in Europe).
EDIT: Also worth noting, and probably the biggest difference between the different lineups, is that Deep Purple has had FOUR lead singers. Rod Evans, Ian Gillan, David Coverdale, and Joe Lynn Turner. Rod Evans was the original, Ian Gillan was Mk. II and everything Mk. VI on, David Coverdale was on III and IV, and Turner V, but he's probably the least known singer of Purple.