I don’t have time to think about this deeply right now, but perhaps you could check
my reasoning and let me know what you think.

The problem with your argument lies in your explanation of cut-off risk being “the
chance of having Down Syndrome given a high risk result”. My interpretation of
cut-off risk would be something more like the lowest chance of having down syndrome
amongst all cases who receive a high risk result (or equivalently, the highest chance
of having down syndrome amongst cases who receive a low risk result). Perhaps
you could call this the marginal probability density at the cut-off. The value you are
calculating is not the cut-off, but rather the average probability density over all cases
with a high risk result, that is, all cases with conditional probability above the cut-off.

Naturally, the cut-off probability must be (substantially) less than the average
amongst cases above that cut-off, which is what you observe in the figures.
Does that make sense?

Cheers,
Mark

Is there something we should know?

Have the clinics got something wrong in the stats for nuchal translucency screening tests?…