Hmmm, what if Milo's spirit, after his eventual death, was transferred into WALL-E, thus resulting in his 'A.I. glitch' that made him who he is?   Thus, Wally is both Milo and WALL-E at once?  He is primarily WALL-E, but has a secondary Milo inside (unlikely the Milo aspect will fully come out other than tinkering, building, and exploring).