How is reward per share calculated on PPS payments system? Does it depend on specific block difficulty or average difficulty per day/week/month used? If I have mined two successive blocks with difficulty 10,000,000 and 11,000,000 and there was the same number of shares for each block, will I obtain 10% higher reward on first block than second block? Or rewards will be the same?

If it depends on particular pool, I need answers on questions:

1. What is the most common practice?
2. How common are the pools that use difficulty of a specific block, rather than average?