Inheritance Tax Question

*I am using the BPP 2019 Kit & I couldn’t type the whole question and answer manually so I just copied the same question and answer from an online site, thats why the dates are different*

(b) Marcus died on 10 March 2015. He had made the following gifts during his lifetime:

(1) On 14 January 2006, Marcus made a chargeable lifetime transfer of £290,000 to a trust. The trustees paid the lifetime inheritance tax of £3,000 which arose in respect of this gift.

(2) On 3 February 2012, Marcus made a chargeable lifetime transfer of £420,000 to another trust. In addition to the gift, Marcus paid the related lifetime inheritance tax of £96,250 on this gift.

(3) On 17 March 2012, Marcus made a gift (a potentially exempt transfer) of 30,000 £1 ordinary shares in Scarum Ltd, an unquoted investment company, to his daughter.

Before the transfer, Marcus owned all of Scarum Ltd’s issued share capital of 100,000 £1 ordinary shares.

On 17 March 2012, Scarum Ltd’s shares were worth £5 each for a holding of 30%, £9 each for a holding of 70%, and £12 each for a holding of 100%.

The nil rate band for the tax year 2005–06 is £275,000, and for the tax year 2011–12 it is £325,000.
Under the terms of his will, Marcus left his entire estate to his wife.

Required:
Calculate the inheritance tax which will be payable as a result of Marcus’s death.

Answer:
When they calculate the IHT liability for the second CLT they take (325,000-290,000) as the available nil rate band rather than (325,000-275,000). Why did the old nil rate band suddenly increase, could you kindly explain me this please sir?
Can you please help me? I went through the notes but couldn’t figure it out. Please help me sir