I believe that, strictly speaking, the 'first 13 months' bit is commonly a fudge by employers. I must admit I haven't read the guidelines for a long time, but when this came up several years ago, it was said that employers don't have to open MPF accounts for employees who are working in Hong Kong for up to one year (and the employees who have come here to work for a year or less do not need to have or contribute to MPF accounts). This was intended to apply to people working on short contracts of up to one year in length, and if someone is taken on with a longer contract (or with the expectation that the person will serve for more than a year), then the short-term worker exemption really ought not to apply and the employer should set about opening an MPF account as soon as practicable after the employee has commenced work.
In practice, the fudge happens and employers give themselves a blanket exemption from opening (and hence paying into) an MPF account for the first 13 months, regardless of how long they expect their employee to stay.
Employers often sell this strategy to new employees as a good thing, as the employee gets more take-home pay for the first year. In reality, however, the employee is losing out on the employer's contribution.