Employee Contract Proration When Start Date & End Date are Mid Month

We are trying to determine how Contract Projected Pay calculates partial months when the Contract Begin Date and Contract End Date are not the beginning/end of the month. We need to be able to consistently estimate someone's deferred compensation when a payment term of 12 months is selected.

An example Employee and their Projected Pay is below. We do not understand how the monetary value is calculated for the first and last monthly amount, given that the contract begins/ends in the middle of the month. Also, why the full months do not equal the Contract Total Amount divided by 12 months (e.g. the amount listed under Monthly Rate)