How much does soil type vary in a garden?

This may be a bit of an idiot's question, but I've done the "soil in a jar" test (digging down a couple of foot for samples) for various areas I have been working on. The soil almost everywhere is mainly around 60% sand, 30% silt and 10% clay, however in one specific tiny area it is something ridiculous like 80% clay - I thought something was up when the area refused to drain at all, whereas everywhere else is well drained.

I have a fairly small garden in the scheme of things so I am quite surprised by the variability. I know soil can vary around a garden, but I am just a bit surprised considering the soil is the opposite of the rest of it.

I'm not in an area known for clay either. I'm guessing it's one of those weird things.