Piwik seems to mix up visits from different visitors (no replies)

Hello,

I have a website where you can log in if you have an account. The data for each account are saved on another database : iduser, name, address...
And when there is a visitor who log in on the website, the iduser is saved in Piwik as a page custom variable .
So, in this way, I know how many visits have been done by each user in my database.

But, I get very weird data on Piwik: I get some visits with several idusers, sometimes until 50 differents idusers in one visit.
And it's impossible. I check the value assigned to my custom variable and I'm sure that only the iduser can be assigned to it.

So, I did a test. With an account, I did a visit of one hour on my website, and I wrote down all the URL I visited and the hour.
Then, I checked on MySQL and there is no just one new visit with my iduser the previous hour, but 12 !
And I checked every visit one by one: I can see all the URL I visited with the matching time, but dispatched among the 12 visits !
Moreover, I can see others idusers saved with each visits, with anything to do with my iduser O_o

So, it seems that Piwik, in some way, mix the visits from different users as one visit.
But how is it possible ? Or what can I do to correct the data ?

Thanks