I agree that ad market or the buyers affect the CTR big time, especially for my entertainment niche. If there's a sale, or new movies coming out, I will have higher earnings, higher CTR.
Traffic is more or less a constant factor. During high season, higher traffic leads to much higher earning, lower traffic leads to "high" earning. During ad market's low season, higher traffic leads to medicore earning, and lower traffic leads to dismal earning.
My strategy is to boost my traffic no matter what happens, so that I can ride high during great ad season, and survive OK during the slow season.