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This likely means that something other than a database query is causing the memory usage to fluctuate, for example an expired cached item that gets regenerated during the page load, or an HTTP API request that sends a request (you can see this in the HTTP API tab). This sort of PHP processing isn’t easy to identify so you’ll need to do some digging.
See https://github.com/johnbillion/query-monitor#related-tools for some related tools that might be useful.