为什么 nmap 显示相同的端口有时显示为已过滤，有时显示为关闭？

Question

当我扫描智能计量设备的端口范围 4050-4060 时，所有端口都报告为关闭：

root@abc:~/rvm/dlms# nmap -p 4050-4060 192.168.1.242

Starting Nmap 6.47 ( http://nmap.org ) at 2016-04-08 15:09 CEST
Nmap scan report for 192.168.1.242
Host is up (0.0038s latency).
PORT     STATE  SERVICE
4050/tcp closed unknown
4051/tcp closed unknown
4052/tcp closed unknown
4053/tcp closed unknown
4054/tcp closed unknown
4055/tcp closed unknown
4056/tcp closed lms
4057/tcp closed unknown
4058/tcp closed kingfisher
4059/tcp closed unknown
4060/tcp closed unknown
MAC Address: 00:0F:93:00:C8:E5 (Landis+Gyr)

Nmap done: 1 IP address (1 host up) scanned in 5.40 seconds

当我扫描端口范围 4050-4059 时（仅几秒钟后），一些端口被报告为关闭，一些端口被报告为已过滤：

root@abc:~/rvm/dlms# nmap -p 4050-4059 192.168.1.242

Starting Nmap 6.47 ( http://nmap.org ) at 2016-04-08 15:10 CEST
Nmap scan report for 192.168.1.242
Host is up (0.0038s latency).
PORT     STATE    SERVICE
4050/tcp closed   unknown
4051/tcp filtered unknown
4052/tcp closed   unknown
4053/tcp closed   unknown
4054/tcp closed   unknown
4055/tcp filtered unknown
4056/tcp filtered lms
4057/tcp closed   unknown
4058/tcp closed   kingfisher
4059/tcp filtered unknown
MAC Address: 00:0F:93:00:C8:E5 (Landis+Gyr)

Nmap done: 1 IP address (1 host up) scanned in 5.44 seconds

有人有什么想法吗？