Why is the processor % different in Activity Monitor vs Resource Monitor?
Resource monitor shows CPU from Processes and Services on two different graphs. (In your example, 32% + 37%, which means a total 69% CPU).

As for SQL Activity Monitor, it shows CPU utilization for the machine on which SQL is installed. (In your example, 64%, which is not that far from Resource monitor.)

I use a query to get the CPU utilization on my server. I run a job on my server and raise an alarm if I get an average over 90% for the last 10 minutes.

Here's my query:

DECLARE @ts_now bigint = (SELECT cpu_ticks/(cpu_ticks/ms_ticks)FROM sys.dm_os_sys_info); 

-- Top 10 : for the last 10 minutes
SELECT TOP(10)  100 - SystemIdle as TotalCPU,
           DATEADD(ms, -1 * (@ts_now - [timestamp]), GETDATE()) AS [Event Time] 
FROM ( 
  SELECT record.value('(./Record/@id)[1]', 'int') AS record_id, 
        record.value('(./Record/SchedulerMonitorEvent/SystemHealth/SystemIdle)[1]', 'int') 
        AS [SystemIdle], 
        record.value('(./Record/SchedulerMonitorEvent/SystemHealth/ProcessUtilization)[1]', 
        'int') 
        AS [SQLProcessUtilization], [timestamp] 
  FROM ( 
        SELECT [timestamp], CONVERT(xml, record) AS [record] 
        FROM sys.dm_os_ring_buffers 
        WHERE ring_buffer_type = N'RING_BUFFER_SCHEDULER_MONITOR' 
        AND record LIKE '%&lt;SystemHealth&gt;%') AS x 
  ) AS y 
ORDER BY record_id DESC;