They are upgraded versions of the earlier moves most of the time.
Like the upgraded version of Scratch is Slash, and Ember Flamethrower. That accounts for increase in power / accuracy and added effects.
Power and Accuracy is measured in Stars.
Taking the Ember - Flamethrower comparison as an example:
>Ember has 4 Stars in power and 6 Stars in accuracy, with the added effect of Burning and thawing Pokemon. It's range is Enemies upfront only.
>Flamethrower has 7 stars in power and 5 stars in accuracy, with the added effect of Burning, thawing and hitting Pokemon from a distance. It's range extends to every Pokemon in it's line of sight.
As you can see, while Ember has better accuracy and PP, Flamethrower is much more powerful and ranged, effectively looking like an upgrade.
Sources: Ember and Flamethrower
Hope I helped!