I have long suspected that Microsoft adCenter is over reporting conversions. Here is the confirmation from my adCenter reporting:
I am guessing that the purchaser visited the ‘thank you for your purchase’ page (which contains the conversion tracking script) 5 times, for whatever reason. I can’t think of any other way this situation could occur – the conversion tracking isn’t set up to take account of multiple purchases in one transaction. How difficult would it be to only count the first visit? Google can do it.
Being cynical, perhaps the over reporting suits Microsoft? But it makes it much more difficult for me to assess the real effectiveness of keywords and ads. Another good reason to concentrate my efforts on Google Adwords instead.