Spark's default recommendations as to calories are based on time (the number of minutes and days) and some broad assumptions as to exercise intensity. These assumptions are fairly conservative, and reflect a relatively low exercise intensity, which is appropriate for most people starting out.
However, as you continue to exercise regularly, your fitness will increase and you will probably be able to burn more calories per minute than Spark assumptions. But you will also likely have a better idea at that stage of how many calories you are burning.
The option is there basically to cater for more advanced exercisers.