Not getting the expected results from conditional formatting

I've been working on this for the last hour and can't figure out why it's not working. I'm using Excel 2010 and I've attached the spreadsheet for reference. For the Questions table, I'm trying to apply a conditional formatting rule to that table. I'm using this formula: =$J7 > (COUNTA(B7:B41)*0.5). Essentially, if >50% of the students miss this question, then apply the formatting rule (so I think). If the value in J is greater than 8, then the entire row should be formatted, right? In my spreadsheet, I have 16 students, so half would be 8. As you you can see from the attached spreadsheet, in column J I have a varying range of values. The formatting is really, really weird. For some values, the entire row is filled in; for others, only two of the cells are filled in. But, if the value in J is > 8, then the entire row should be filled in, but it's not for a number of rows. Why is this happening? Thanks in advance!