Questions on Twitter

After weeks of intensive use of Twitter there are still questions left. This week I tried to find out how many pages I can go back with users who tweet a lot, like thousands and thousands of tweets. I checked two users and could only go back in time for 160 pages, with 20 tweets on a page that’s 3200 tweets. On page 160 the button ‘older’ is disabled. Next thing I wonder is where are all the older tweets? Are these lost somewhere in cyberspace? Could there be a back-up somewhere? I tried to use the Wayback machine but when I click on a link for 25 december 2007 for @silvertje I get an error. When I try to do the same for @marinalog on 13 december 2007 I get a page without any mark-up (lack of css) but with significant less tweets (5,969 in stead of 20,122 as I write this). This is the only page I can get from this user, although I can click on other usernames who have much more tweets today. Why can I only view these days in december 2007?