The difficulty changes are designed to keep blocks being found at a steady pace. So when the total hash rate of the network goes up, and blocks start getting found faster, the difficulty will go up so that blocks will continue to be found at the same rate.
That's how we can predict when the next block halving will be. It's all about math. And the reason your reward goes down, is because there is more hashing power finding about the same amount of blocks. So if you don't increase your hash power at the same rate the network/pool hash rate increases, then your share of the pie will get smaller.
So the lower hashrate is specific to me, correct? What I SHOULD see is the same hashrate, the same payout just less often.
Your hash rate should be stable - it will fluctuate slightly depending on individual worker luck - but if you had about 2 GH/s last week, then you should still have 2 GH/s this week. This is regardless of difficulty increases, increases in the pool hash rate or increases in the network hash rate.
That is the easy bit - your own hash rate is a relative constant.
Now, assuming your own hash rate remains constant (and you not adding in lots of usb miners - so you stay at 2 GH/s):
1) If the pool hash rate increases but the total network hash rate stays the same (so in other words, other existing miners move their rigs over to slush's pool)?
Then you should expect to find blocks more frequently - but your share of the payouts will go down slightly. Net effect is you earn the same over a week.
2) If slush's pool hash rate remains the same but the overall network hash rate increases?
Then you should expect to find blocks less frequently - but your share of the payouts will remain the same. Net effect is you earn less per week.
3) More typically, if slush's pool hash rate increases and the total network hash rate increases proportionally?
Then you should expect to find blocks relatively consistently - but your share of the payouts will go down. Net effect is you earn less per week.
