Stock Rover calculates the dividend yield by dividing the forecasted yearly dividend payout by the current price. For example, in the case of Comcast (CMCSA) using data from September, 8th, 2018, the last quarterly dividend payment was $0.19, so the forward dividend yield is (4 * 0.19) / 36.17 = 2.1%. The forecasted yearly dividend payout is listed in Stock Rover in the Dividends Per Share metric.
It is possible that the other sites are calculating dividend yield using a trailing dividend value, causing the difference between their values and ours.