ArcGIS Pro not accepting negative volume value for Cut Fill

I am using the Cut Fill tool of ArcGIS Pro to calcuate volume change in DTMs over time. I am hoping to visualise these on a map where Cut (volume removed) is blue and Fill (volume added) is red. I was hoping to assign my own values for the primary symbology by classifying them in manual intervals. When I look at the attribute table, there is positive and negative values (as is to be expected). However, when I try and enter a negative value for the symbology classification, ArcGIS states that "The upper value specified is out of range. The default will be the minimum value." which is 1.0.

However, as you can see, there is both positive and negative values for VOLUME in the attribute table

But when I try and enter a negative value for the manual classification, ArcGIS prevents me from entereing a value below 1.0.