June 2015 Past Paper

Extract from question #1 on inheritance tax:

Zoe’s husband had died on 25 July 2007, and 20% of his inheritance tax nil rate band of $300,000 for the tax year 2007-08 was not used.

Extract from answer:

Available nil rate band
Nil rate band- Zoe $325,000
Husband (325,000 x 20%) 65,000

My question is why isn’t the nil rate band transferred to Zoe from her husband 20% of the 300,000 which was the nil rate band at the time of his death?