GS, i think that because the dinamic distribution is based on free bw not total bw. so for example in your case you had 2 dl, each taking 0.1 so you had 17.8 aviable not 18, when you tell it to use 50% it calculates 50% of 17.8 thats 8.9 then when you tell the other one to take the rest it uses the 9.1 left.
Exactly as you said.
So, if you arre using 50% in one, you will have to select 100% at the other because, whatever it calculates percentage from free bandwith, if you select 50% and is is taken by another download, it will remain for the one you set up first.