I am confused!!!

I have a table (PROPERTYHEADER) with a PRIMARY KEY (PROPID).

When I do:

update propertyheader
set globalupdate = '01/01/2001'
where propid = 30022225;


Statistics show:

PLAN (PROPERTYHEADER INDEX (RDB$PRIMARY59))
PROPERTYHEADER INDEXED READS 1
PROPERTYHEADER NON-INDEXED READS 26283 (one for every record)


Why am I getting so many NON-INDEXED READS????

It looks like it is using my Primary Key based on the Plan!!!