Why do some covenant weapons overcome drastic design changes over time? I'm talking specifically about the fuel rod cannon and the beam rifle. 

In Halo Reach the fuel rod cannon has this solid design, in Halo CE it has that purple design (or the same design as in Reach with Anniversary textures), in Halo 2&amp;3 it has a more skeletal and bulky design and in Halo 4 and H2A it has the same design as in Halo Reach. I'm assuming this is just because the devs have decided to start using a different design since Halo Reach, but maybe there is a lore related reason behind this?

Talking about the beam rifle though, in H2&amp;3 it had the triangular design and in H4&amp;5 it has a completely different "crocodile" design. Since this design did not appear in the H2A campaign, then I'm assuming that it has changed during the events between H3 and H4, but for what reason?