nmap port scan

nmap sT 옵션을 사용하여 port scan을 하다가 궁금한것이 있습니다...

첫 scan할때와 이후에 scan할때 결과가 다르다는 것인데

첫번째 port scan 결과................

Host is up (0.027s latency).
Not shown: 973 closed ports
PORT STATE SERVICE
79/tcp open finger
135/tcp filtered msrpc
445/tcp filtered microsoft-ds
683/tcp open corba-iiop
873/tcp open rsync
1045/tcp filtered fpitp
1046/tcp filtered wfremotertm
1047/tcp open neod1
1060/tcp open polestar
1147/tcp open capioverlan
1216/tcp open etebac5
1233/tcp open univ-appserver
1583/tcp open simbaexpress
1717/tcp open fj-hdnet
1718/tcp open h323gatedisc
2049/tcp filtered nfs
2107/tcp open msmq-mgmt
2608/tcp open wag-service
2869/tcp filtered icslap
3077/tcp open orbix-loc-ssl
3878/tcp open fotogcad
4444/tcp filtered krb524
7025/tcp open vmsvc-2
8090/tcp open unknown
8701/tcp open unknown
10002/tcp open documentum
13783/tcp open netbackup
Nmap done: 1 IP address (1 host up) scanned in 1.47 seconds

그이후 두번째 port scan 결과................

Host is up (0.015s latency).
Not shown: 989 closed ports
PORT STATE SERVICE
135/tcp filtered msrpc
445/tcp filtered microsoft-ds
1045/tcp filtered fpitp
1046/tcp filtered wfremotertm
1047/tcp filtered neod1
1717/tcp open fj-hdnet
1718/tcp open h323gatedisc
2049/tcp filtered nfs
2869/tcp filtered icslap
4444/tcp filtered krb524
8090/tcp open unknown

Nmap done: 1 IP address (1 host up) scanned in 1.47 seconds

첫번째만 open된 port가 많이 나타나고
이후에는 계속 두번째 로그처럼 적게 나타납니다.

그러다가 몇분뒤에 다시 scan하면 첫번째 로그처럼 많이 출력되구요..
결과가 상이한 이유가 뭘까요...?