The only thing I can think of is that the Plasma Bolt technology continues further than the Electric one at the highest tech levels. Basically, I use the Electric weapon until I've reached the highest technology levels - only then do I use the Plasma (b/c it's better).
Something related that I don't understand: why the name change in the middle of the technology tree? The weapons do the same thing as the lower tech ones - why not just keep incrementing the roman numerals?