As I understand the definition, a corpus lutea is the vacant spot occupied previously by an ovum within the ovary, now filled by a yellow body that is clearly not an ovum.  Couple with that the term "degenerated", which means whatever was there to replace the spot that once held the egg has now decomposed somewhat, and you have a total misrepresentation of a potential litter offspring.  You are literally counting "chickens before they hatch".  It would be a foolish move, indeed, to include these masses as viable offspring, because they do not consist of such material.  They are spots filled with some type of glandular liquid, but are hardly the genetic material needed for the production of healthy, viable offspring.  One would be much more accurate to count only the existing ova available for fertilization to calculate the number of litter.

Explain why counting placental scars produces an overestimation of actual litter size?

Counting placental scars within the uterus of a breeding female is a time-honored wildlife biology technique for estimating populations in mammalian species. In mammals that have litters originating from multiple eggs, each offspring has its own placenta; the location where the placenta is attached to the lining of the uterus leaves a permanent scar after the offspring is born. Theoretically, dissecting an animal and counting the scars in her uterus should tell you have many offspring she has delivered. This, however, does not tell you litter size unless you also know how many litters she had. If the last litter was not too long in the past, then the newest set of scars may look different, which does help with accuracy, but counts obtained this way may still result in overestimates because offspring that were aborted or reabsorbed will still leave scars.