Because they are written for the standard case and every business is full of exceptions. Part-time patterns, mid-year joiners, unpaid leave and long-term absence each need a rule, and if the policy does not have one somebody invents it in the moment.
Those invented rules become precedent, and two years later nobody can say what the policy actually is.
Decide three things explicitly: how entitlement accrues (up front or monthly), how it pro-rates for part-year and part-time, and what happens to accrual during unpaid leave and long-term absence.
Write them down before configuring anything. Most disputes come from a case the policy never addressed rather than from a disagreement about the rule.
Either, as long as pro-rating and leavers are specified.
Yes, and they usually should for part-year staff.
Yes, provided for in Books at current rates.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.