I interest myself in delta robots and after viewing many pictures about such architecture, something feels strange to me.

According to this interestering calculation page, it appears that the best positionning precision is obtained when the bicep arm is almost as long as the forearm.
And I think it also allows to reach the highest speed but it's just a feeling....

But all delta robots I have seen are the bicep arm really shorter, half or less, of the forearm leg. What is the reason?

Another question is about the precision. The given page gives a +/- value but such definition is relevant for linear displacement but here, may I understand that the uncertainty area is a disk?