The one mature female rabbit gave birth to one baby female rabbit at the beginning of the month. So there is a total of two female rabbits (and several males).
The new chart shows this. The entry under month 3 shows the total number of rabbits at the end of that month. There are: 1 mature rabbit and 1 baby rabbit which is one month old.
If you look under month 2 you see how many mature female rabbits there are at the end of month 3.
N | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
Rabbits | 1 | 1 | 2 |