I don’t understand why you don’t understand this.
a lot of basic numeracy gets forgotten by adults.
very few adults can do for example 4739 divided by 21 quickly. Because they don’t need to very often in their daily life and as everyone knows if you don’t use it regularly you forget it.
in practice in a job you use a calculator.
the maths skills needed by graduates in a role at the treasury do not require you to do division on a regular basis. If they did people would revise how to do it and do it.
they do require you to be able to build mathematical models using a range of assumptions and work out error bars based on changing assumptions and statistical analysis.
the skills are in the same area (maths) but they are miles and miles apart.
in the same way an advertising copywriter might not pass a grammar test (wtf is a fronted adverbial anyway?)