How may there be more people than there could be for very long, if there'll never be more people than there could be?

Question

Source: Benatar, David. Better Never to Have Been (2008 1 edn). pp. 165-166.

I don't understand the semantic distinction between 1 and 2 beneath.

OVERPOPULATION

At the time this is being written, there are about 6.3 billion people alive.³ Very many people think that this is too many—that we already have an overpopulation problem. Others think that unless we do something about population growth (or unless something is done to it), there will very soon be far too many people. Even those who do not think that the population sizes projected for the next century or two would be too big certainly think that there is some population size that would be too large. Nobody can reasonably deny that there is some population that would be too large, or, in other words that there could be overpopulation.
  The notion of overpopulation is normative, not descriptive or predictive. [1.] There never will be more people than there could be,⁴ but there may well be more people than there should be. But how big a population is overpopulation? This question can be asked of either (a) the cumulative population or (b) a population

 ³  A billion is 1,000,000,000.
 ⁴  However, [2.] there may be more people than there could be for very long.

at any given time. The latter question—how many people may there be at any given time?—is the usual one. This is because the number of people living at any one time can impact on the welfare of those (and later) people,⁵ or (some environmentalists would argue) impact on the planet. Anthropocentrically, there may not be enough food to go around, or the world may simply become too crowded. Environmentally, the ecological ‘footprint’ of a very large human population may be too great.⁶ Thus the usual concern is to avoid having too many people around at one time or within some specified period. That is a reasonable concern. However, as I have indicated, we can also ask the population question about the cumulative population—how many people may there be throughout time?⁷ In so far as most people can make sense of this question, it is a function of the concurrent population, the (possible) duration of humanity, and the circumstances of each period of humanity. In other words, their answer to the question ‘How many people may there be throughout time?’ is calculated by summing,⁸ for every consecutive period of humanity’s possible duration, the answers to the question ‘How many people may there be within this period?’ However, as I shall show, the question about cumulative population size can be asked and answered in other ways.

Answer 1

The first one states that it isn't possible to go beyond what is possible, ie. You cannot create people without them adding to the maximum definition of how many could be made. You could call this the 'blatantly obvious' rule, if there are 100 people then having 100 people must be possible.

The second one states that you could be in an environment where 100 people can survive indefinitely, and that is how many you could have there 'for very long'. You can go over this number, but you will not be able to remain above it forever. Think of a spaceship creating enough water for 100 people. You could put 200 people on there, but it would eventually decrease to 100 before stabilizing.