Yes, there needs to be a place for the variability of the brace timber width. The legs of the triangle as well as the hypotenuse are fixed lines and can't be changed. So the variability has to be on the inside of the triangle.
We make the pocket and the housing larger by at least 1/4" then the length of the brace shoulder so that if the brace timber is oversized as it is rough sawn, then there is some way to compensate for the extra width.
If the stock is wider the brace shoulder length will be longer and if the pocket and housing are cut to the drawing size then the brace will not fit. Doing it longer eliminates the risk of the brace not fitting.
You can size your brace stock and make the pockets perfect if you wish but that is a lot of extra work.