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1 theory: G saves our real clicks and money and it shows us only small portion or speculate our earnings and after some time we see our real values. maybe G doesnt want to show real values.
For example, we see high and low earnings. 1 click = 5 cents, 1 click = 2 dollars. Sometime we see high ctr, sometime we see very low ctr
2 theory: G use seasonal effects in different manner. G can save our money in dec-feb and it may release after some time. So that we will see very high earnings in following months ( high because it includes our previous earnings )
These are just theories of mine