The Vary symbology by attribute tab does not honor the changes in the sample size parameter of 'Advanced symbology options' in ArcGIS Pro.

Workaround

As a workaround, follow the steps below:

1. Save the layer with the incorrect symbology as a layer file.
2. Open the JSON in a text editor, and modify the maxSliderValue and maxValue to force the visual variable settings to display the true maximum for the dataset.

Steps to Reproduce