I want to know why the Transaction Summary sampled-events list shows many more events than the Web Vitals view for the same page or transaction.
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That difference is expected. It is not a sampling bug by itself.

The Transaction Summary sample list includes all operations for that transaction name, including client-side navigation events. These navigations are often much more frequent than full pageload events.

The Web Vitals experience is driven mainly by pageload data (and related vital measurements). Navigations can increase the Transaction Summary count without adding the same volume of Web Vitals samples.

To compare like with like in Transaction Summary, filter samples with:

transaction.op:pageload

If pageload counts still look high but Web Vitals is sparse, some pageloads may be missing the measurements needed for scoring. See Why are some transactions not showing up in Web Vitals?.
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