In a discussion with a local youth who collects stamps, he posed this question. Hw was looking at the 2009 edition of the U.S. Postal Service Guide to Stamps which uses Scott numbers and values. Some of the early se-tenant issues (examples are 1972 1456-1459 and 1973 1480-1483) have the uncancelled joined block of 4 with a lower value than each of the 4 stamps separated. He asked if he should separate his block of 4 so that the stamps would be worth more! I understand that Scott's places a 20 cent minimum on a single issue, but it would seem logical to me that for a se-tenant issue, the whole should always be equal to at least the sum of the parts! Oh, yes, I convinced him to keep as a block of 4.
The Postal Service Guide to US Stamps is based on the Scott Catalog, and the Scott Catalog does indeed show lower values for many blocks and/or se-tenant set of stamps than they do for each stamp separated individually. In fact, there are even situations where some of these modern blocks of stamps are worth more used than mint! Not that the stamps are particularly valuable, but many multiples were saved by collectors in mint form and few saw postal use. However, if you tried to use those stamps today, they would probably be worth less as the term "used" typically applies to contemporaneous cancellations and not cancels that were applied 10, 20 or 30 years later.
The stamps you recite are from the early 1970's and to take the issue to the extreme, look at the 2-cent National Parks Centennial stamps (Scott 1448-51). As each separated stamp is given a minimum catalog value of 20 cents, you would think a block of four would be worth 80 cents, but in fact it's assigned catalog number 1451a and only worth a mere 25 cents!
The reality is that catalog values aren't meant to give a true value to a common stamp. The minimum catalog value takes into account a dealer's cost to maintain his store, and his overhead, and his cost to keep on-hand the inventory of stamps a collector may want. In most cases stamps sell for much less. It's all supply and demand. If a collector needs one or two stamps to fill spaces in his album, no matter how common, he may be willing to buy it above catalog value simply to accomplish his goal while others may wait until the stamp is found at a deep discount.
The other issue to consider is that in dealing with a used stamp that may have a catalog minimum of 20 cents, most will know that the re-sale will probably be a couple of cents (if that); just because someone may have accumulated 10,000 copies of that same used stamp, doesn't mean that you can apply a $2000 value to it; as they will still only sell for pennies.
That's why catalogs are often referred to as a "Guide" Just because an item may be listed as being worth a set amount doesn't mean it's worth that on the resale market. The true value all comes down to what a buyer is willing to pay and what a seller is willing to accept to make the deal go through.
An analogy I use quite often is that you can go into a new car showroom and see the perfect car with a $30,000 sticker price but few would expect to pay that, as most would want a discount and a rebate and any other promotional incentive to get the car at far less than list (catalog) price may suggest. Same general principle holds true for stamps or coins or antiques or any other item of value.