Keep in mind some companies in the index will pay dividends quarterly, some annual, and some semi-annual. This is part of why they vary quarter to quarter, and Q4 is typically the largest.
I believe that in general, the larger the company, the more frequent the payment. So a smaller REIT might pay annual, while GE would pay quarterly. Based on my holdings, it also seems foreign companies are more likely to pay annually, although that is anecdotal.