SHARE BASED PAYMENT

I need some clarifications on estimating the number of employees expected to leave. I’ll address this by citing different scenarios and how their treatments, in my opinion, seem not to be consistent.

Scenario 1

Year 1: 20 employees leave & the entity estimates that of 20% of the 500 employees will leave during the 3-year period.
Treatment: 80% of 500 employees were estimated to remain; 20 employees that left were not considered.

Year 2. A further 20 employees leave & the entity estimates that only 15% of the 500 will leave.
Treatment: 85% of 500 employees were estimated to remain; again the further 20 employees that left were not considered.

Scenario 2

Year 1: It was estimated that a total of 22% of employees would leave prior to the end of the vesting period. At each reporting date within the vesting period the situation was as follows:
Year. Emp leaving in the yr. Further leavers expected prior to v. date
1. 8. 18
2. 6. 4
Numbers of employee were 200.
Treatment: In the first year 26 (8+18) employees were estimated to leave
In the second year 18 (8 (for 1st year that left) + 6 (for 2nd yr that left) + 4 (expected to leave)) employees were estimated to leave.

The confusion

In scenario 1 the employees that left in both years were not considered in determining the number of employees estimated to remain. Whereas, they were considered in the second scenario.
Year 2 in the second scenerio considered leavers in year 1 in determining the number of employees estimated to remain. But it was no so treated in Year 2 in scenerio 1.

Can you please throw light on the right treatment?