The first value is an exponential moving average of your hash rate. Therefore, this value is a good measure of your current hash rate after a few minutes. The second value is an average over the whole run time and it should be equal to the first value after a few hour, if the pool connection is stable. One Radeon RX 480 8GB does about 23.8 MH/s with stock clocks and your card seems to be close to 21.7 MH/s (you have to run it longer).
The WU (work units) are used to calculate the number of HW errors. Let's assume your card mines with an average of 20 MH/s and has an error rate of 2%. Then the expected WU value would be 20/m * (1-0.02) = 19.6/m. The hardware errors are normal, but they aren't shown by other Ethereum miners. A defective card typically produces 10 times more hardware errors then a working one.