Its probably pretty late answer.

However, as someone who has worked with this in the past and spoken to Google. They weren't much help TBH. I ended up talking to their supervisor, and we figured it out together. The reason is that Analytics works off a different methodology from Ads. Basically, 1 track on last click and the other 1st. Don't ask me why. What's even crazier is that on your analytics dashboard, it will even attribute it to Ads. You would think they would be the same, but nope. Put it down to Google departments not talking to each other.