Graded vesting means that portions of a single option grant will vest on two or more dates. The IFRS 2 requirement is explained in IFRS 2.IG11:
For example, suppose an employee is granted 100 share options, which will vest in instalments of 25 share options at the end of each year over the next four years. To apply the requirements of the IFRS, the entity should treat each instalment as a separate share option grant, because each instalment has a different vesting period, and hence the fair value of each instalment will differ (because the length of the vesting period affects, for example, the likely timing of cash flows arising from the exercise of the options).
That had been FASB's proposal as well. However, after considering the comments on its current exposure draft, FASB has now decided to give entities the choice of recognising the expense on a straight-line basis. Treating each instalment as a separate grant has the effect of recognising more expense up front.