Am I being thick 
I doubt it.
What it shows is that N% of the telephone lines for the postcode of interest are connected through that particular PCP.
Fictitious examples:
(1) Postcode AB1 2YZ is found in the database linked with PCP13 and shows a "percentage lines" value of 100.
Meaning: all existing telephone lines for the AB1 2YZ postcode area are connected via PCP13.
(2) Postcode CD3 4WX is found in the database linked with PCP24 and shows a "percentage lines" value of 63.
Meaning: sixty three percent of the existing telephone lines for the CD3 4WX postcode area are connected via PCP24 (and the other 37% of the existing lines are connected via another PCP).
Does that help?
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100% Linux and, previously, Unix.