Possible bug? Error in calculation giving unexpected result

I've made a spreadsheet to demonstrate the problem I'm having. It's pretty simple. The sum of one row is divided by the sum of two other rows (divided by 2). The answer comes out wrong unless I break it down into two smaller steps. Could someone tell me whether I'm doing something wrong or if this must be a bug?

I tried to put the spreadsheet into the OneCloud so you can view it and hopefully download it.

Edit: I've attached the file to this post in case that's easier for some.
Excel miscalculation.xlsx