OK, despite being a little too fond of thinking of myself as being more astute than the average person, I have to admit that it didn't occur to me initially why the Jobs Credit Scheme was set at 12%. I wondered why that particular number was picked, thinking it seemed arbitrary, but now I know why.
Since the CPF employer contribution rate is set at 13% of an employee's salary, it's immediately obvious why the Jobs Credit Scheme cannot credit more than 13%. Otherwise, companies could use phantom workers to make money off the spread between the CPF employer contribution rate and the Jobs Credit rate. Ergo, the Jobs Credit rate is set just shy of 13%, i.e. 12%.
It just goes to show that the average coffeeshop owner has more business acumen in gaming the system than me, the postgraduate trained engineer. Sighz.