Mass Property - Volume - Reading Incorrectly

I have a model that I've made an internal cavity and then using the intersect tool I am able to measure the internal volume at different points. 

I came a across an error where my full volume is less than a partial volume. Right when I'm near the top (see image below) my total volume is 431.86L. When I go right to the top, my total volume become 431.83L 

Partial Volume:

Full Volume:

SolidworksModeling And Assemblies