When I put your hops in, I get different numbers. I get 43.7 IBU. When I change the batch volume to 6G, I get 40 IBU.

Comparing the tool to my recipe, I get 69.1 in the tool and 67.2 in my IPA recipe. Adjusting my batch size to 5G adjust the recipe bitterness to match the tools calculation.

I tweaked numbers and found trub loss affects estimated IBU as does BH Efficiency. A bit of trial and error, it seems the tool assume an efficiency of 60%. The higher your efficiency, the lower the IBU. Since my efficiency is 63.1%, my calculated IBU is a little lower