Your link doesn't work?
A 3/4-16 should be a 3/4-16 on both? Are you measuring tip to tip with your calipers? Got any pics?
Last summer, I bought one of those cheap lathes, had to make a whole new spindle from scratch. The concentricity was .011. Piece of crap.
I'll see if I can find pics, I bet your is made by the same company.
When I made a new spindle, I cut 1-8 TPI.