It's based upon:
Advertiser bids (which can vary seasonally - many advertisers pause campaigns around holidays)
Number of advertisers bidding (less competition = lower bids)
How well your account converts for advertisers
Which ad people are clicking on (can be a huge difference between the first and last ads on a page)
... to name a few things.
FMI and video:
https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=160525
PS - you do know that Adsense program policies forbid posting your EPC and CTR? You're allowed to mention your earnings, but not the details.