Utilisation and Conditional Formatting - not quite working...

I have a table of resourcing information as per the below (the 'FTE' column is %age of the working week that each person is going to be spending on the different tasks) and I have created a utilisation measure which sums the FTE values divides by the number of weeks and then multiplys by the number of people (this is so that I can use it to get a utilisation across more than one person).

The problem is that when I add conditional formatting to a matrix table it doesn't quite tally. If I set the conditional formatting to over 90% then it works for any values over 89% if the time period is 1 week, anything over 88% if the time period is 4 weeks, but only anything over 95% if the time period is 10 weeks???

Utilisation formula:

So I know that it is something to do with the number of weeks / the date that I select but I can't figure out why it would be doing this? Any ideas where I could start looking?

Thanks, Paul