Perhaps that was down to the usual lazy effects work in much of the film.
As I said before they had a really nice model and hardly filmed it so maybe when the came to compose that shot they changed the way it fired to make it work with the few painting and model elements they bothered to put out.
We may never know because asking for a straight answer (even if we could) from the guys at the ranch is a hit and miss affair.
I don't envy the work of anyone who wants to rework the sequence so it matches the firing in ANH as it would mean either lots of new paintings or a scratch model build.
So it's a matter of take your pick.
That is either a really bad kludge of a shot or that IS the way the weapon works (which allows anyone reworking ANH to make it fire anyway they want).
Personally I never liked the way the weapon fired in the first place.
There is a big dish with a nipple in the middle so it just makes more sense to me that the smaller beams focused on the nipple thing (technincal term) and the large ray fired out of it and was focused by the dish.
Having everything fire out into space and meet there looks suitably science fantasy but doesn't really make much sense visually.
I'm sure someone has written a umpty thrumpty paged manual which explains how the superlaser works but I don't buy it myself.
That's my soapbox moment over ;-)