Species richness: Why does the more species at one plot imply the more species are missed? Species richness: Why does the more species at one plot imply the more species are missed?
This is what my lecture notes say.

The more there are species that have been observed only once, the more
one can assume that observations of species have been missed.

This sounds counterintuitive, but perhaps the idea is that if one plot has "such many", then it's likely that the rest have also "such many".
I.e. e.g. if one plot out of 20 plots has 32 species, then in the worst case it's possible that the remaning 19 plots also each have ~32 new species. And the total would be e.g. 608 species, while having most species "only once observed" would suggest that it's 32 + something.

This is not about "more species", it is about "more rare species".
If you keep finding species that occur only once (only in one plot or maybe only one individual), that suggests that you might be dealing with a community that is rich in rare species. So you can assume that if you would establish new plots between the existing ones, you would keep finding more of those rare species that you've missed before.
See also: Species–area relationship and how that affect your study design in terms of plot size/number.
