Reducing/lowest amount of payments on account

Hello,
Whats the difference in calculating these?

Q1: Eva’s income tax and class 4 NIC for 2017/18 = 4840
for 2016/17 = 6360
What’s the lowest amount to which Eva could make a claim to reduce each of her payments on account for 2017/18 without being charged interest?

Answer: 4840/2 = 2420.

Q2: Zhi’s income tax and Class 4 NIC are, or are of forecast to be:
2014-15 2015-16 2016-17
Inc.tax liability 25200 27600 18000
Class 4 NICs 4084 4204 3724

Calculate the amount to which Zhi can claim to reduce his self-assessment income tax and NICs due for payment on 31 January 2017 without incurring interest or penalties.

Answer:
For 2016/17:
Current POA (27600+4204) * 50% 15902
Revised POA(18000+3724) * 50% (10862)
————
Reduction 5040

So Why question 1 isn’t calculated like this: 6360 * 50% = 3180
4840 * 50% = 2420
——-
Reduction 760
…what am i missing? is it because there is a reduction in POA only when expected liability is lower and in q1, the income tax and class4 NIC liability is already calculated?

Or is it because a the POA can be in full amount of the liability, but lowest half of it?