Whilst it is true that the saber in A New Hope does seem to snap on and snap off, it should be noted that the saber Luke uses in A New Hope is not the same saber featured in Return Of The Jedi - he of course loses his original saber in The Empire Strikes Back in the battle with Darth Vader when his hand is cut off, and has constructed a new (and entirely different) one by the time Return Of The Jedi starts.
To be honest, I think the behaviour of the sabers in A New Hope is, to put it generously, inconsistent - even in The Empire Strikes Back, in which Luke is using the same saber as he is given in A New Hope (albeit with a few modifications), the behaviour of the blade is much more what we are used to - i.e. it stays the one colour, it extends and retracts as we expect, sound effects are those you would normally associate with a lightsaber, etc - and does not behave in the erratic way it is depicted in A New Hope.
In my opinion, the unusual behaviour exhibited by the lightsabers in A New Hope is simply down to their actual properties not being fully established yet by Lucasfilm in terms of how they should look, behave and sound, and then by the time the second movie was under production things were much more clearly defined, giving us the lightsabers we all expect today.