SMF 1.1.5 and default template, ads break on right side

[mnymkr]

I am using a slightly css modified default template,

when no one is logged , and you view  topic, the ads on the right fall to the bottom of the screen instead of staying as a tower on the right.

If I am logged in this does not happen.

you can see it at www.mychemistrytutor.com/forums

what can be causing this?