I think you're being a little to literal and, in a way, missing the point. The terms used are not meant to be taken literally. "Penetration" is just another word for "damage" and "Protection" sounds better than "Target Number" or even "Armor Class". The terms havce no real direct correlation to the shields' functioning.
Keep in mind that even without shields, a ship has a "Protection" rating. It may only be 5, but it's still there.
The stats that affect how the shields are functioning are Treshold (which is likely what Picard was refering to with the lasers bit) and the Shield Strength track. The former determines if a weapon can penetrate the shield while the latter determines how the shields are holding up ("shields at XX%").
Former Decipher RPG Net Rep
"Doug, at the keyboard, his fingers bleeding" (with thanks to Moriarti)
In D&D3E, Abyssal is not the language of evil vacuum cleaners.