Share -based payments

Hi!!!,

Can anyone tell me that while recognizing expenses for the share based payments, when there is a change in Fair value

then why the decrease is ignored and in the case of increase too only the incremental part is considered and not the revised fair value . I mean what's the logic behind that.

 for eq. if the fair value considered earlier was 15 for recognition of expenses , and after that there is a reduction in exercise price ie. originally FV before reduction is 14 and after reduction it is 17  then we have to consider 15+3 ie. 18 as a FV to recognize. not the 17 why???