Mobile Visibility 25% lower than desktop - any opinions?

Hey SEO friends!

I would like to hear your opinion on a certain topic, and that is the relationship between desktop and mobile visibility.

We track the visibility via the tool "Sistrix". Until 2019, the desktop and mobile values were almost identical. Since September 2019 (core-update) the mobile visibility is about 25% below the desktop visibility (Desktop: 50, Mobile: 36)

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Do you have any idea what this might be caused by? I could not find a specific source of error so far. Have you had similar experiences? In my other projects the ratio of desktop and mobile visibility is almost identical.

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Thank you !