IHT: Gross CLT

Hello I hope you don’t mind another question.
From Rev. Kit BPP (new editoon FA 2017) q 189.

CLT (14/1/2008) = 315000

CLT(3/1/2015) = 491250 (gross)
What was the lifetime IHT paid on this later CLT, if Donor paid IHT?
(NRB 2014/15 = 325000)
IHT
avail. NRB (325000 – 315000) = 10000 * 0% = 0
481250 * 20% = 96250.
So, why 20% when donor pays at 25% IHT? the explanation in the kit is as follows:
“even though Marcus(donor) pays IHT due, since the gross CLT is given, to work out the tax he pays this is 20% of the excess over the available NRB…?