The two different tabs are the same chords, the first is an "easy" one for beginners, it'sand is correct according to videos of Ritchie Blackmore and Steve Morse. It's an inversion with the 5th as the lower note, and the root an octave high. You can get away without it in the bass position, as your bass guitar provides a strong root (and rhythm) throughout.
The second example is the more normal form of the powerchord, and it has the root note in as the bass as well, which doubles up the octave.
Both contain the 5th, not the 4th, so it looks like you were assuming the lowest note wasif playing on your own this may be the root, which is an easy mistakebest way to makeget a bigger sound.
In terms of chord choice, music doesn't need to remain within any framework, however classic rock is very much grounded in blues scales, and Ritchie Blackmore has long been keen on breaking out of expected musical theory into interesting directions (see any of his later progressive stuff for examples!)