Different visits with other tool


A friend told me, that he has over 100 visits a day. I analyzed his site with Piwik for a few days. Comparing the numbers of piwik and the other tool (Webanalyzing Tool from 1und1.de) show me that there is a big difference of visits.
I installed the piwik-code on the two index-sites of his Page. Even the index-pages do have different visits. What´s wrong here?
I normally use Piwik on CMS-sites where I put the code in the template and everything works fine. But this site is something special :wink:
Maybe he counts every visit on the webspace - but then the visits on the index.html should be the same, or not?