Can't really explain the difference in a way that would make since but they are definitely not just different lines of the same snake (like black pastels and cinnys)

Personally I dont really know how to explain it to you but I wish I could.

Mostly I'd just say lessers are MUCH lighter, tend to be a more goldish color where as the Mojaves have browns/grays and such in them as well. Look at a picture of a lesser and a mojave next to one another and they're VERY different. Now if you want to know something that's close and harder to tell.

Look at the difference between a butter and a lesser...that's where I'd say more people get confused, but personally, neither mojave or butter can match a lesser.