I also notices that.
From my testing it kinda looks like it takes the width of the elements inside the Horizontal Box that use Size / Fill as if they were set to Size / Auto (so their normal width), sums it up, then from this is subtracted the width of all elements set as Size / Auto, then it takes that result and divide it by the number of elements inside the HB, then based on that value and the ratio from fill value it applies the width of each element with Size / Fill. This is in the case Size / Desired is used and so it is doesn’t have its size restrained. When it is, it will use that restrain as max value and keeps the same ratio for those with Size / Fill, after from that value is subtracted the width of all elements set as Size / Auto.
Which is wrong if that is how it does it.