Kaplan study text test ur understand 2 ...shared based payment

An entity has a reporting date of 31 December. On 1 January 20X1 it grants 100 share options to each of its 500 employees. Each grant is conditional upon the employee working for the entity until 31 December 20X3. At the grant date the fair value of each share option is $15. During 20X1, 20 employees leave and the entity estimates that a total of 20% of the 500 employees will leave during the three-year period. During 20X2, a further 20 employees leave and the entity now estimates that only 15% of the original 500 employees will leave during the three-year period. During 20X3, a further 10 employees leave. Required: Calculate the remuneration expense that will be recognised in each of the three years of the share-based payment scheme. I didn't understand why they just removed 20%=100 employee n what about the 20 employee in 20X1 ... Then again in 20X2 15% n ignore the 20 employee... Then 20x2 they took both 20+20+10 ...plz explain am confused ?