Strange visit in visitor log

I have the version 1.82 of piwik and I have tried to import Log of urchin.
The log import fine, but the result of piwik and urchin are not the same (number of visit, time per pages …).

I have checked the Visitor Log, and some visitors have many action in 0s (for example 17 action in 0s).
If I check, the details of the visit, I have a long time between some page visited. For example:
I have Two pages visited the 08 May 2012 at 20:35 and 15 pages visited the 15 May between 17:40 17:45.
Why this pages are grouped in the same visit ?
I have try change the visit_standard_length parameter, but I have the same error.
The website http://demo-log-analytics.piwik.org have the same problem:

Dim 24 Jun - 21:55:21
FAI: Inconnu
6 Actions - 0s

  1. 301 Moved Permanently Dim 24 Jun 18:23:44
  2. 301 Moved Permanently Dim 24 Jun 20:21:44
  3. 301 Moved Permanently Dim 24 Jun 20:49:48
  4. 301 Moved Permanently Dim 24 Jun 21:10:19
  5. 301 Moved Permanently Dim 24 Jun 21:10:19
  6. 301 Moved Permanently Dim 24 Jun 21:55:21

Have you a solution for this problem ?
Best regards
Fabrice