Grainfather ABV Calculation

[vortura]

Is it just me, or are the ABV calculations on the Grainfather site just plain wrong? Full disclosure: total noob, so it's probably me. But this recipe of mine for example:
https://community.grainfather.com/recipes/513089
I'm using Mangrove Jacks M44 yeast there, with 81% attenuation entered. But the recipe stats show OG 1.047, FG 1.006, ABV 5.4%.

By my calculations though, 1.047 to 1.006 is (47-6)/47= 87% attentuation.

What's going on here?

 

[MikeCo]

The actual attenuation will vary depending on the recipe ingredients and mash temperature. The manufacturer listed attenuation is 77 - 85%. The Grainfather site appears ot be using the value for the middle of this range. Recipes with a lot of simple sugars and low mash temperatures could certainly attain attenuation higher than 85%.