BecomeMe

For the same reason a human in a bad automobile crash may be conscious and yet be immobile - the brain releases its own mix of painkillers along with other chemicals such as adrenalin when your body gets badly damaged.

That's sometimes why you can get a big gash in your arm running or fighting or even moving furniture around, and might not immediately notice until you move into a lower state of excitement and those chemicals clear out of your blood stream.

Someone can be very badly injured, but since they're in shock they might not be able to tell how badly they are hurt until the local environment changes.