How are trace impedances summed together for a single trace?
As a rule of thumb, a trace 2h wide on a substrate of thickness h and er 4.2 (like FR4) has an impedance in the 50 ohm ballpark. The Saturn PCB figures look like what I'd expect. 

Are you sure you've given Altium all the correct data, it's unlikely to be that wrong by itself. Check any defaults that are lurking around.

The trace does not have an 'overall' impedance, unless its length is so short, or the frequencies are so low that the concept of impedance doesn't really matter. If it's short, do a weighted average. If it's long, you would need to model it as a length of 45 ohm line, followed by a length of 43 ohm line, followed by some 39 ohm line. Is there any reason to keep changing the width?

However, the change in impedance as you go along the trace is sufficiently small that the reflections caused by those impedance change are likely to be negligible for most purposes.