Alarm manager, using wakelocks to ensure all code runs

Question

I'm looking at using the Alarm Manager, and read this in the developer docs, which I don't really understand.

"If your alarm receiver called Context.startService(),
it is possible that the phone will sleep before the
requested service is launched. To prevent this, your
BroadcastReceiver and Service will need to implement a
separate wake lock policy to ensure that the phone
continues running until the service becomes available."

• http://developer.android.com/reference/android/app/AlarmManager.html

I am specifically asking for which situations it could possible that the phone will sleep before the service is launched (as this is the part I don't comprehend)? Is it dependent on how fast the phone can execute statements? ie. it calls startService() which opens another thread and so the original thread could complete its work before the service has been made available?

Thanks