Assume you have an empty glass in front of you. Now, you start pouring water into the glass. Every now and then I'm going to come by and take a look at that glass and measure how much water you've got in it. The first time I come by, you've got the glass only 1/5th of the way full. I mark that down and pay you for 1/5th of a glass of water. I come by a while later and you've got the glass 2/5 full. OK, I mark it down and pay you for 2/5 of a glass of water. This continues until the glass is full of water. Once your glass is full I pay you for a full glass of water, but I also now open a valve at the bottom of the glass so that water drains out as fast as you're pouring it in. In this fashion, your glass will always be full of water and every time I stop by, I see you've got a full glass and pay you for it.
Eventually, you stop pouring water into the glass. However, that valve is still open, so the water is draining at the same rate you were pouring. When I stop by the next time, I see you've only got 4/5 of a glass, so I pay you for that much. The time after that, the glass is only 3/5 full, so I pay you for that. This continues until the glass is empty.
In my story above, every time I stopped by is equivalent to the finding of a block. I see how much water you have in the glass and pay you for however much is there. My story assumed I stopped by at even intervals of time, so you'd always know when to expect me.
So, how does luck play into it? Well, every time I stopped by, I made sure to calibrate that valve on the bottom of the glass. If I stopped by more often than expected, I'd mark you down and pay you. If I stopped by less than expected, the valve would clog up and the water you were pouring in would slosh over the edge of the glass and be wasted. When I finally did stop by, I would notice you had a full glass, and pay you for it, then fix the valve so that the pour rate equaled the drain rate.
OK... maybe I shouldn't try explaining things after I've been drinking
.... Andddd...gone. 