It said: "The traditional method involves assigning members a work unit comprised of a particular range of nonce, the number that blockchain miners are computing for. Once the pool member completes the work on the assigned range, he places a request for a new work unit to be assigned."
From what I understand, the nonce range is the same for all miners and in order to proof their work, miner need to send the share with the difficulty fixed.
So I don't understand the interest to increase the difficulty for a miner. If he keep the minimum difficulty, why would he reduce his lucks to proof his work with a higher difficulty?