TOP n sometimes gives wrong number of lines (SSRS 2008 SP1)

Hello all,

a workmate has created a dashboard report which contains (among other items) a top n chart item. The user can select the value of n using a report parameter. We have observered that for some values of n and specific values of the other parameters the chart shows the wrong number of lines (bars), e.g. I choose to select TOP 5, but the chart shows in some cases the TOP 6.

Is such a behaviour already known?

I will try to create a minimal sample for reproducing this behaviour during the next few days.

Thanks in advance and best regards,
Gerald