Ad size has no bearing, if that is what you are asking. Sometimes it is just what kind of ads appear in that size allocation which gives a different metric.

IE. win some, lose some.

It doesn't seem so probable that just this ad should go down so much when I have another two ads that are the same size and the ctr stays the same?

Seasonal shifts in the available inventory are the most likely reason. When you get the high payers it just happens that that position that gets the clicks.

It can very well depend on the ads that are pushed out by advertisers. Maybe a new movie, tv show, or a cool product came out that people wanted to know about.

In the beginning ads are bid higher to make sure that they are seen, then the bid prices gradually drop off as time goes on, since the ad buyers know that people already know about the product.. At the same time users are less likely to click on the ads that they have seen before.

So you get cycles of lower CPC and CTR at the same time with the various product cycles. Until the next new thing comes out. Seasonal shifts matter big time too as competitors are bidding up the ad spaces during certain times of the year.