Because EZ-Filing automatically calculates the total of all the paychecks received during the 6-month income-inclusive period and then calculates the average monthly amount during that period.
More specifically, for paychecks (that is, wages, salary, tips, etc rather than other (nonpaycheck) income), the Statement of Current Monthly Income ... window shows the average monthly income during the income-inclusive period, whereas the Edit Employer and Edit Paycheck windows show the amount of one or more individual paychecks received during that period.
If, for example, your client had five identical paychecks of $1000 during the income-inclusive period, the amount in the Statement of Current Monthly Income ... window would be $1000 (per paycheck) X 5 (paychecks) divided by 6 (the number of months during which those paychecks were received), thus, $5000/6 or $833.33, whereas the other windows would show the amount of each individual paycheck ($1000).