Just wanted to chime in, although I don't know the math either. R0J0's alteration changes the speed of your zooming (your original lerp had referenced its own position as it moved, in effect changing it from a linear interp to an exponential one), but the curving zoom path is still there.
The curving zoom is a perception issue. Basically, your scrolling is progressing at a steady rate (assuming R0J0's version), as well as your scaling. However, what you perceive is that in a zoomed out state, you are scrolling much slower than when you are zoomed in, even though the actual speed has stayed constant.
To get the effect you are looking for, your scrolling speed needs to be modified by the scale: faster when zoomed out, and slower as you zoom in.
Edit: Quick hack - not the final answer since the scrolling is now jerky, but you can see the idea