Why does the buy limit order trigger above the limit?

Are you gauging this based on average cost basis or from the filled order details? Because if you take average cost basis, most brokers will include the comms too, which increases the 'filled price'. For example, buying 2 shares of VOO with a limit order of 300 on FSMOne, the order might filled at 298.6, but because of the comms (8.80/2), the average cost will be increased to 303.