Grading

The coins (depending on the specific year) are finished differently and/or produced differently. For example, coins in sets from the 50's and 60's were done as Proof-Like. PL coins normally have a much more mirrored finish to the flat fields than circulation coins do. Therefore the worst grade you will see would be PL63 or PL64 for a given year. Which means a PL65 is the average, common grade for these types of coins. Whereas for a circulation finished coin an MS65 is an unusually high grade and priced accordingly. Starting some time in the 70's the mint changed the process and the coins in sets were then called Brilliant Uncirculated (BU). Some people continue to call them Proof-Like. ICCS now grades them with the Numismatic Brilliant Uncirculated (NBU) comment to distinguish them from the circulation coins. A couple of years back the mint changed again and now they just put circulation coins in the sets which, personal opinion, is why you're seeing a relatively high number of coins graded MS66 and MS67 the last couple of years. Specimen coins have always been a different beast and usually have a completely different look to them; again depending on the year.