What determines EPC?

[Raymond]

On my site there are 3 main ads that generate 95% of the adsense revenue. I call these A, B, C ads. A has the highest CTR and C has the lowest, and all these 3 ads are on the same page.

Back in December all A, B and C had roughly the same EPC, but the past couple weeks EPC of A has gone down quite dramatically while C remains roughly the same.

What causes this discrepancy in EPC? These ads are on the same page, so they should be loading ads with similar relevancy. If I am smartpriced, why only applied to a particular ad, which happens to be the one with highest CTR?

[Genuine1]

EPC depends on:
a)Advertiser competition for webspace which varies
b)how well your clicks convert
c)Your traffic source which also varies
d)smartpricing
e)google algo which varies
f)the mood of your visitors in that they may have bills/hols/other that effect buying decisions and therefore effects smartpricing/conversion
g)advertisers budgets change
h)mfa infestation on various niches changes fast and they pay little
i)seasonal effects
j)traffic relevancy
k)ad wording changes
l)real advertisers turning off content ads or doing advertising on other networks due to the mfa problem leaving lower competition for your webspace for the remaining advertisers
m)any number of other things...