This detail has not escaped me, being a fan of the X-Wing / Tie Fighter series and trying to do my own interpretation of a low-poly 3D model of a Snowspeeder...
I've gotten many reference images, and found the most movie-accurate ones show a clear inward angle on the cannons. This means the convergence point would be extremely close and make it impossible to hit anything beyond a certain distance as the shots would splay outwards beyond that distance.
I could make my own argument that their designed to converge at the longest effective range, after which the energy in the blast starts to dissipate, but is there any "canon" reason? (So to speak...)
But even then, it looks to me that the convergence distance on those would be mere meters in front of the craft if you traced them forwards... which I've done below.