Name one reason why throughput and data rate are different?

Throughput is what your ACTUAL speed is at the given time of testing your line speed, the data rate is what this figure is, for example your ISP may say you have an 8Mb maximum connection speed but the actual throughput will vary with time of day+how many people are using the network, so you may see a throughput with a data rate of 4Mb or less if there is heavy usage on your ISPs network during the day+maybe an improvement late at night when there is less internet usage in your area..