Take for example something which you can purchase at any grocery store across Alaska. Something such as mayonnaise. You typically only need a couple of bottles to last you and your family for more than a month or so. Which is good because mayonnaise has a fairly short shelf life. It is not like a bottle of wine which could last you years if it remains unopened. When it comes time to replenish your families mayonnaise supply you perform your due diligence as a rational actor in the market place and attempt to find exactly which place has the cheapest mayonnaise. You find that the cheapest single bottle of mayonnaise can be purchased at Fred Meyer's for three dollars. However, you can also find at Sam's Club that you can get a pack of three bottles for seven fifty (two dollars and fifty cents a bottle).
Well you want to get your mayonnaise as cheaply as possible and two fifty a bottle is the cheapest in town. So you purchase the three pack at Sam's Club. Then return home and proudly proclaim to your family the great deal you got on mayonnaise, hoping that they will look on with envy for your inhuman bargain hunting skills. Unfortunately, they are less than impressed and point out that the household typically only uses two bottles of mayonnaise before the expiration date on the three pack would pass. You argue that is fine because it is still only two fifty a bottle so the mayonnaise is still cheaper than if you had bought two individual bottles for three dollars each. An undeniable point they all admit. However, they point out that the amount of utility they will receive from the three pack is equivalent to only two bottles of mayonnaise (thanks to the expiration date). So while the price per unit is lower the price per util is substantially higher. Let us assume that one bottle of mayonnaise provides use with one util of utility up to two bottles. After which we receive no benefit at all.
Purchasing two bottles at Fred Meyer's for three dollars each also equates to purchases two utils for three dollars each. However the three pack at Sam's Club costs two fifty a bottle but costs three seventy five per util of utility (7.50/2) since you can only use and thereby benefit from the two bottles. So in fact it was more expensive to purchase the three pack at Sam's Club than it would have been to buy the amount you needed from Fred Meyer's. Since you are not really purchasing mayonnaise but the benefit which it could provide you with. Obviously there are a variety of ways in which you could still come out ahead from the Sam's Club purchase such as selling the additional bottle for one fifty one or more. Or you could change your diet to include more mayonnaise (maybe that would be a benefit?). However, let us imagine for this argument that no such options are available. Well now it should be clear what the Sam's Club effect is. It is taking into account price per unit without considering the marginal benefit we receive from each unit. Basically it is purchasing more than you need because it is cheaper that way. Often when people are bargain hunting at Sam's Club they fail to take into account how they will actually use their five gallons of soy sauce or in this case all that mayonnaise.
Oh... and the book I mentioned earlier isn't real... Sam's Club might not be either... I would have to check...