Why do I have holes in my pages?

Why do I have holes in my pages?

Postgres 8.4.12

 "select ctid from big_table" on my master shows that pages have
"holes" in them.

Here is example for page 431665:

 (431665,2)
 (431665,5)
 (431665,8)
 (431665,11)
 (431665,14)
 (431665,17)
 (431665,20)
 (431665,23)

Current DB size is 400 GB and it takes up 2.7 TB (on a 6.6 TB filesystem).

So what happened to rows 1, 3 and 4 and so on?

I have to size a database server for next year's budget, and I will
have to explain to my mgmt why we have 400 GB taking up 2.7 TB.  Help?
 Would appreciate a pointer to the appropriate section in the manual
if this is documented.

Thanks,
-at