PA restriction on TO/LDG charts
 

All performance charts for take off and landing are limited to -1000 feet PA, I would like to know the reason.
It is possible to find 1047 hpa at SL, not common but could happen once in a while in areas such Alaska. Not to mention places like Bar Yehuda Massada in Israel where field elevation is around -1250 feet and I guess, quite common to operate on PA below -1000.
On the other hand, JAR 91.144 limit aircraft operations when QNH is above 1050 hpa, BUT A320 QNH window admits settings up to 1100 hpa.
As far as I know altimeter readings cannot be lower than -1000 feet.
As you can figure out I´m not an specialist, but would like to know....
It might be a reason that neither the FAA nor the JAA require flight test for certification porpouses for PA below -1000 feet.

Any answer will be very much appreciated,
REGARDS