Shape not in interpolation curve not preserved

I'm using an interpolation curve as a way to import a 2D geometry into my model. Figure1 shows the shape of the data I wish to import (as shown in Excel). However when I import this data, saved as a .csv file, the curve loses it's wiggles and becomes a straight line (Figure 2). This is not good. The imported curve has the same range in x and y values so I know the import is working it just has lost detail. I've set the relative tolerance to 0, 1, and 1,000,000. It doesn't have an effect.

Any input?