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I have been handling Facebook Advertising for close to 4 years and I am witness to similar scenarios on a daily basis. Hence, I believe I would be able to answer this question correctly.
Since I have limited information about your ads, I am assuming you are running both of these ads under the same ad set.
Ads in an ad set compete against each other for delivery. Facebook rotates all your ads until there is a clear winner.
Broadly speaking, this is the Facebook algorithm at work & will happen every time you have more than 1 ad under a single ad set. Even if you put 1001 ads in 1 ad set, there will be 1 ad that will outperform other ads & will be the only ad that Facebook shows to your target audience.
To understand the factors affecting different CPCs for ads, you can consider the following factors:
1. CTR of the ad
2. Engagement on the ads
3. Creative & Message on the ads and its relevance to your targeted audience.
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Best of luck!