iptables -n doesn't display numbers in all cases for prot column

iptables -n doesn't display numbers in all cases for prot column

[Marcin Mirosław]

Hello!
I'd like to ask you about such situation. When i display rules without 
using "-n" i'm getting:
# iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
  pkts bytes target     prot opt in     out     source 
destination
     0     0 ACCEPT     ipencap--  any    any     anywhere 
anywhere
     0     0 ACCEPT     ipv6 --  any    any     anywhere 
anywhere
     1    88 ACCEPT     icmp --  any    any     anywhere 
anywhere
    94 13850 ACCEPT     udp  --  any    any     anywhere 
anywhere
   801  632K ACCEPT     tcp  --  any    any     anywhere 
anywhere

It's ok. When i add "-n" switch i'm getting:
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
  pkts bytes target     prot opt in     out     source 
destination
     0     0 ACCEPT     4    --  *      *       0.0.0.0/0 
0.0.0.0/0
     0     0 ACCEPT     41   --  *      *       0.0.0.0/0 
0.0.0.0/0
     1    88 ACCEPT     icmp --  *      *       0.0.0.0/0 
0.0.0.0/0
   101 14402 ACCEPT     udp  --  *      *       0.0.0.0/0 
0.0.0.0/0
   905  651K ACCEPT     tcp  --  *      *       0.0.0.0/0 
0.0.0.0/0

As we can see protocols "ipencap","ipv6" are shown as number but 
icmp/tcp/udp wasn't displayed as number of protocol.
$ grep "tcp\|udp\|icmp" /etc/protocols
icmp    1       ICMP            # internet control message protocol
tcp     6       TCP             # transmission control protocol
udp     17      UDP             # user datagram protocol

# iptables -V
iptables v1.4.12.1

My question is: why some protocols are always displayed as protocol name 
even i use "-n" switch?
Thanks for help.
Marcin