Frosted 1966 Quarter

Why is this quarter so heavily frosted, particularly the obverse. The fields, although you can't really tell from the photos, are/were absolute mirrors. I have lots of PL sets from the mid 60's but they mostly resemble polished nickel. Also have lots of business strikes but they tend to be white.

I read an old tread where someone referred to them as first strike PL coins. Is that correct and does that mean all PL's had intentional frosting which eventually disappeared over time from die polishing or some other process.

This is what keeps me scratching my head.

Cheers and thanks.