Potting compost

[MereDintofPandiculation]

Advice from people who know about gardening wanted! It’s usually said that you can use as potting compost a mix of two parts garden soil to one of garden compost. Why is this is? Is it

a) to make the garden compost go further
b) to get nutrients from the soil that are lacking in garden compost
c) some other reason?

And how can they be so prescriptive given the vast range of garden soil and garden compost?

As I understand it, the reason given for adding stuff to true compost is typically it needs to be mixed with other material to create a stable mix. Healthy soil is one of the materials that can be added to help drainage, water retention, and aeration, and for less shrinkage. Healthy soil has a mineral content, clay, silt sand, and organic matter (while true compost is organic matter).
Soil is an option. Taking soil up from the garden is a disturbance and has an impact though.
There's an article below that discusses soil, compost and potting mixes.
On the observation that it's prescriptive to state ratios, yes but it's probably just a general rule of thumb and a suggestion, taking into account soils are different, and composts are different depending on the context.
https://horticulture.co.uk/compost-vs-soil/

The RHS are encouraging people to compost and make their own potting mix with their own soil and their own compost where possible because it's a local, sustainable, affordable, solution.

Problem is, you're potentially introducing all sorts of nasties which could be present in your garden soil, into compost which is sterilised during the bagging process

I use garden compost for potting sometimes, but find if I mix in soil (mine is clay/loam) it has more 'heft' (I'm sure I've seen a pp use that word!) and doesn't dry out as quickly. But if you don't have spare soil I think just garden compost is great.

I find pure garden compost dries out quite quickly in pots, it's too free draining. In this garden which is heavy clay, I've mixed it half and half with soil from the garden in pots. If you think about it, it's much the same as what you do when you add the organic matter to the soil, you end up with a mix of soil and compost.

I suspect that 1/3 of the mix being compost is enough to give the nutrition needed for most plants while they are in pots, and the soil will start to get them used to the conditions they will be in once you plant them out in your garden. So in that sense the variation in soil composition between gardens is irrelevant to the prescription, on the assumption that most people will end up planting what they grow in their pots in the garden the soil came from.

I make my own potting compost with garden soil, home made compost and if I have it, leaf mould. Home made compost can be too rich for some ants; garden soil is generally too heavy in pots and ypu don't get aeration or efficiwatetjng; snd leaf mould has little in thd way of nutrients bu provides a more open texture to the mix. I usually make it I equal parts. If I have no leaf mould I sometimes use sharp sand or coir especiallyif I'm sowing seeds. It seems to work fine for my pots and seedlings but you do have to watch for weeds.