Go read the website I linked to. NASDAQ publishes their methodology. I'll quote it for you here:

  OUCH Measurements of Order Latency
 
    * The time it takes to accept, process, and 
      acknowledge or fill an order.
    
    * Measured as the complete round trip for an order 
      or cancel transaction leaving a customer network 
      port, through the co-lo network and application 
      firewalls to the matching engine and back.
    
    * Measured in the production environment during a 
      full trading day including opening and close.

As you can see, they are including the time it takes to acknowledge and fill the order. It is the complete round trip time, and does include transit latency through the co-located network (which I thought was excluded).

They are clustered together because it takes roughly the same amount of time to determine that an order can be matched (filled) or that it can't be matched (acknowledged, on the book.) Either way, it has to be processed by the matching engine.