Googlebot crawls but doesn’t fetch pages

I’ve been seeing a surge in the number of "Crawled – currently not indexed" pages in search console recently. Screenshot below:

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When I look in detail to URLs one by one, I see that some of them are crawled but not fetched, like below:

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Site’s average response time to googlebot seems to be in the range of 200-300 ms. What can be a reason for these crawls but no fetches?