I noticed that many of my Uncirculated mint sets from the 1970's have some rather poor looking coins in them - tarnished, bag marks, etc. I don't understand why they look that way, considering the fact that the coins are sealed in original mint packaging. On the other hand, the mint sets from 1984 and up are in remarkably better condition. What is the reason for this? Did the mint improve quality standards for the packaging and/or the the coins used for the mint sets in 1984?
Did you buy the sets new or from a dealer? If you bought them from a dealer you might have gotten ones that were handed around or already cherry-picked through.
rbf - are you asking about mint sets or Proof sets ? Either way, the explanation is probably the same. The simple answer is not all mint/Proof sets contain coins that are equal in quality. If you are unhappy with the quality of the coins in your sets - it's a simple matter to trade the ones you have in on new ones that you like. Just visit a dealer or any coin show. You may be able to trade straight across - at most it might cost you a couple of bucks a set.
The early Uncirculated mint set coins were just regular minted coins that the mint sealed in Cello and sold at a premium. I don't think there was very much effort in selecting the coins to use in these sets.
