Hi Jack! Breww uses the user-entered production totals in Settings → Beer duty settings in order to calculate your duty relief. Although Breww suggests a figure based on the total volume brewed under your own licence in Breww, we made sure you can always override it exactly for reasons like this.
If you’re looking for a more in-depth breakdown of your production totals, the duty payment option and the brewery brewed for, you should be able to use the following Raw data explorer share code to pull out the raw data behind what you need 
--- BEGIN BREWW RDE SHARE CODE ---
eyJiYXNlX2RhdGFfY29kZSI6ICJiZWVyX2JhdGNoZXMiLCAiZmllbGRzX3RvX3ZpZXciOiAiZ3lsZSwgdGhpcmRfcGFydHlfYnJld2VyeS5uYW1lLCBkdXR5X3BheW1lbnRfb3B0aW9uLCBiYXRjaF9yZWYsIGJlZXIubmFtZSwgcGxhbm5lZF9zdGFydF9kYXRlLCBkYXRldGltZV9zdGFydGVkLCB0b3RhbF92b2x1bWVfZXhjbHVkaW5nX21lcmdlZF9pbl92b2x1bWUiLCAiYnJld3dxbF9maWx0ZXIiOiAiIn0=
--- END BREWW RDE SHARE CODE ---