I want to know why the event count in the issue header is higher than the count in the All Events tab.

The header count is the lifetime total for the issue. All Events shows only events that are still stored and match your current filters.

The numbers can diverge when older events age out of retention. Errors are kept for 30 days on Developer and 90 days on Team, Business, and Enterprise. After that, the events are removed and no longer appear in All Events, but they still count toward the header total. See Data Retention Periods.

Filters on the issue page can also affect the numbers shown in All Events. Search, environment, or time range changes can change All Events and the graph without changing the header totals. See Issue Details.

Expired events can’t be restored. For plan-specific retention details, see How Long Are Errors/Events Stored in Sentry?.