Does anybody know if there is a reason that the pattern '//jack' has a 
  priority of 0.5 where the pattern just 'jack' has a priority of 0?


well if there wasn't this difference there would be no point at all (in
xslt1) of allowing a pattern to start with // as changing the priorityis
the only effect it has:-)

It is not that the default priority rules specifically say anything
about a leading // it just isn't one of the special simple cases that
gets a different default. There are any number of similar examples such
as
jack[true()]
which also matches the same nodes as jack but with different default priority.

Some early drafts of XSLT1 had far more baroque schemes for assigning
priority to specific pattern forms but I think basically they thought
that if the default priority scheme was not sufficiently simple that you
could keep it in your head, no one would use it and just assign
priorities by hand, so by the time of the XSLT 1 rec it had evolved to
the scheme that you see. Of course xslt2 is a bit mpre complicated again
to cope with the extra node test forms, but it still follows the same
basic structure as in 1.0.

David

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