The one mature female rabbit gives birth to one baby female rabbit during the third month. So there is a total of two female rabbits (and several males).
The new chart shows this. At the end of month three there is 1 mature female rabbit and 1 baby female rabbit for a total of two female rabbits.
If you look under month 2 you see how many mature female rabbits there are in month 4. Since it takes rabbits two months to become mature, every rabbit listed for month n will be mature in month N+2.
If you look under month 3 you see how many total female rabbits there are at the start of month 4.