drumsticks, it works the other way around too.

Sometimes we might go a few days and only find one block, and other days we find a few blocks in one day. In the end, over time it evens out to around 100% of what is expected.

That "over time" bit depends on the size of the pool, this is called variance. The larger the pool the smaller the variance, so that it evens out to 100% of expected in a shorter time.

At the moment I would say we have good months and bad months, but over a year we hit 100% or greater expected return.

When all the bitcoins run out, we will be mining for the transaction fees, but that won't happen for a long time as there are nearly 4 million blocks yet to mine.