Lease Timeout VS HealthCheck Timeout - AlwaysON
The HealthCheck is between WSFC and SQL Server. The timeout is how long WSFC will wait to get data back from the sp_server_diagnostics execution.
LeaseTimeout is the SQL Server resource and SQL Server Availability Group. How Lease Timeout works


There are two timeouts because they are for 2 different mechanisms for health of the WSFC and SQL Server. Both are changed via the WSFC property of the AG resource, picture is shown in the linked article above.