May want to look at this thread--someone was having a similar issue, and there was an issue with previous versions of Tableau, evidently. Make sure you have upgraded to 8.04
Thread is here: http://community.tableau.com/message/230463
Thank you for the hint. I can’t tell if the two problems are related, but I am already on 8.04.
Maybe it's an internal representation issue. Tableau only has 'Number' as a data type; I can only assume it's floating-point. When a computer performs math it may have an answer that can not be represented exactly so a rounding event occurs -- maybe this is what we're witnessing. I dunno, why not ask Tableau support directly and tell us what they say?
Since you're only dealing with two decimal points why not set the Default Properties >> Number Format... to two decimal places?
I think you are likely to be right about the floating-point Toby. The erroneous rounding occurs already at the first decimal when I use more data.
I have contacted support, but have to wait for my local affiliate to process the case to Tableau support. I will give this tread an update when they come back to me.