MAC OSX vs Win XP: Different stats test results!

Hello Peter,
thanks for your quick response - you have hinted into the right direction. I triple-checked my data and - how embarrassing - I actually did have different data matrices. Due to a faulty Win Excel macro that sorted data into groups after data collection, I ended up with different data in my 3 groups in the Win "test" ... All good now :-). Thanks again! 

Best wishes,
Mareen