‘Other’ references "Assets Not Allocated" while blank/NULL items have not been categorized by the perspective yet.
For a given perspective, you should expect to see a value when added to FlexReports so if nothing is showing it likely indicates that the billing data represented is "ahead" of the perspective. In other words, the items listed as NULL or blank may be newly identified in terms of the perspective configuration and the next update of the perspective should pick them up and categorize appropriately.