DR of shields? Why so much higher than ordinary slab of wood?

[Ultraviolet]

The DR of shields range from 5 to 9, from the smallest to the largest.

I assume these are wooden shields (although leather or hide on a frame of wood or wicker seems an alternative.

But B558 lists wood (for doors and walls) to DR 1 per inch. and the "cover DR" table B559 lists even less, DR ½-1 per inch?

So why are shields that hardy? With the realtively low weights of sheilds, they're not made of *that* many inches of wood. I guess a metal band could add to the DR, but these stats are listen for TL0-1 and later, so I think no metals are involved?

And why does DR increase with size of sheild? Aren't they made the same way, of same materials, perhaps a bit sturdier construction to better stabilize the increased surface area, but still?
Sure HP increases with size, but DR?

Perhaps it's a game balance thing, for the optional rules for damage to shields?
Consider a longbow arrow? At thr+2 ST 11, thats 1d+1. On the best of days, that won't penetrate a Heavy Steel Corselet with DR 7. Nor will it get through (or even damage slightly) a Medium Shield, also DR7. Had the shield been steel, it'd have+3 Dr, for DR10, ouch!
Consider a ST 12 geezer with a Broadsword (Sw+1) that's 1d+3 damage, or a range of 4-9? He'll never, ever damage a Large Shield, and only a 2 in 6 chance of chipping a Medium Shield slightly. And forget about it if not using wooden shields. Not even deliberately, by hacking and hacking away at it!

That just seems a bit odd to me. IIRC the old viking tales (which *do* I know were told from one drunk guy to another again and again, untill someone pulled out a block of stone and chiseled down the runes...but still) tells about viking duels, where both parties had a set number of shields. Because shields could and did get destroyed. I believe this gag was used in "the 13th warrior".

But perhaps game testing had shown that using the DR1-2 range for shields, destroys them too quickly. Even though most blows blocked won't damage at all, just those rolls only made by the margin offered by the DB. Few fighters would deliberately go for smashing the enemy's shield.

Or did I miss something, about the DR only being for penetration purposes, to see if an attack blows through and hits the guy holding the shield, but the entire damage is subtracted from the shields HP?