^^pretty much this
for example, every page in a thread is 50posts..
if you have a thread with 151 original posts you have 4 pages (50+50+50+1)
however if one or more posts get deleted in the lifetime of the thread, it doesn't alter the "page counter" or howere you want to call it, it only counts with unique posts and doesn't recalculate for deleted posts...
so in short for this example you could have 3 posts deleted which would result in-> (50+50+48+ghostpage)
i hope this clarifies it a bit for you ...
it's also a known issue and the admins are working on a complete overhaul of the site, so don't start shootin em PM's about it, they already know and it will get solved eventually...
Cheers!