Why computers are users in Active Directory?

I was able to run the following LDAP query against my Active Directory:

ldapsearch -H ldap://$ad_ip:389 -x -D $ad_user -w $ad_password -b "DC=itdrde,DC=local" 
    -s sub -a always -z 1000 "(objectClass=user)" "serviceClassName" "serviceDNSName" "objectClass"

and got the following output:

Name" "objectClass"
# extended LDIF
#
# LDAPv3
# base <DC=itdrde,DC=local> with scope subtree
# filter: (objectClass=user)
# requesting: serviceClassName serviceDNSName objectClass
#

# Administrator, Users, itdrde.local
dn: CN=Administrator,CN=Users,DC=itdrde,DC=local
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user

# Guest, Users, itdrde.local
dn: CN=Guest,CN=Users,DC=itdrde,DC=local
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user

# 7-DEV-DE-DC, Domain Controllers, itdrde.local
dn: CN=7-DEV-DE-DC,OU=Domain Controllers,DC=itdrde,DC=local
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
objectClass: computer

# krbtgt, Users, itdrde.local
dn: CN=krbtgt,CN=Users,DC=itdrde,DC=local
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user

...

see here for full output.

As I can see from output computers are also has objectClass:user, why?