1. The reward of each block is known as it is mined, therefore it is not rando reward and we are ideally only mining the high reward blocks.

2. What I suspect happened is that a high reward block was solved just before Wafflepool did, and I believe that currently Wafflepool has their minimum shift time for one coin to 9 minutes - Unfortunately in some cases that means that a dogecoin 800k reward block is found within 30seconds leaving us with 8 mins 30 seconds mining say a 200k reward block, which is obviously not as good.