Alright, so I was playing on the PBE, and I noticed that in a way, Jarvin's lance looks like Aatrox's sword. Also, they both expand similarly. This made me start to think, so I came to these forums, then I saw a thread where it had a link to the AMA( I missed it) and the window picture. So currently, I have a, somewhat possible, most likely not, theory
My theory is that, after Aatrox helped Demacia, they created a lance modeled after his weapon, and Jarvin eventually received that lance.