determining an xproc extension attributes

in section 3.9 Syntax Summaries it states;

'It is a static error (err:XS0008) if any element in the XProc
namespace has attributes not defined by this specification unless they
are extension attributes.
'
then in section 3.8 Extension attributes it says;

'A processor which encounters an extension attribute that it does not
recognize must behave as if the attribute was not present.'

Both of these statements seem to create a contradiction.

How can a processor 'know' it is not recognizing an unknown extension
attribute ?

J