Tooltip into the wrong position

Hi,
I have a some labels on my solution which have to show a tooltip.
One of these labels (the one with the bigger tooltip) is displayed on the top instead of on the bottom.

Here is how it appears:
[attachment=2]popup cut.PNG[/attachment]

This is the involved html:
[attachment=1]tooltip html.PNG[/attachment]
As far as I can understand, the position is calculated and put directly into the div properties (left: 1476 and top: -2352).

Here is the related CSS:
[attachment=0]css.PNG[/attachment]

The others are properly working.

Any idea on how to fix that problem?

tooltip html.PNG