Match request 1170, Impressions 506, Viewable impr 254 in AdMob

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I have this stats on AdMob:
Match request 1170, Impressions 506, Viewable impressions 254, CTR 0.40%

Should I be worried about the ratio between impressions and viewable impressions?
What is your ratio?

What about match request vs impressions? Is it OK?

I have 2 activities in my app. In the first one you are editing the image and
in the second one you are reviewing saved images. When you open the second activity,
an interstitial add is preloaded. When you press back button the add is shown.