NF_IP_PRE_ROUTING?

[scarab@echostar.pl]

Hi
I wrote simple linux firewall (nf_hook based). Below  is part of that code:
------------
static struct nf_hook_ops       lpf_nfh;

lpf_nfh.hook = lpf_hook_fn; 
lpf_nfh.hooknum = NF_IP_PRE_ROUTING; 
lpf_nfh.pf = PF_INET;  
lpf_nfh.priority = NF_IP_PRI_FIRST; 

nf_register_hook(&lpf_nfh); 
--------------
 
I have question. It should get only incoming packets so why it takes outgoing 
to? If i will use  NF_IP_POST_ROUTING [4] then it should get outgoing packets 
right? I found that diagram in "Linux netfilter hacking howto":

--->[1]--->[ROUTE]--->[3]--->[4]--->
                          |                   ^
                          |                    |
                          |              [ROUTE]
                          v                    |
                         [2]                [5]
                          |                   ^
                          |                    |
                          v                    |

Can anyone tell me why it works that way?
Thank you

PS. Sorry for my poor english