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In at least one place it is -rather, it's a combination of factors, one which is mixed materials - cement render over lime render, sitting on a base where water sits (because the path was laid with waterproof adhesive over concrete), so water wicks up, then bridges the join between a modern cavity wall and solid old wall, hits old plaster again, and blows out the new more waterproof plaster laid over the top.
It's going to be fun fixing this lot.
In other news, I'm going to have a go at installing a lead flashing apron later.
NotThamesWater
I doubt that your damp problem is caused by the mixing of different materials.
There could be many different reasons for it.
But that is not one I've heard of.