I think the traffic estimator is taking a higher quality score KW and giving you a general range. I think that the CTRs seem over-ambitious, myself.

If you haven't already run the account, how do you know it's on the low end? Were you just predicting more?

You don't have to be at the Top Spot of course. Position 3-5 are still normal to target and will give you a lower CTR, lower CPC & more traffic. Depends on what your KPIs are, I guess.