Exactly - if it does not break at 3, you know it will break at 4 since you tested it.
If you alter the decision tree, you'd have no idea whether floor 4 breaks eggs, you only know that floor 5 breaks eggs and floor 3 does not.

If it helps, extend your tree with the result - if you have more tries then eggs, you'll reach a result once the last egg breaks, eg. down from 1 you get the result 0, down from 2 you get the result 1,... and to the very right, next to the 10-throw you omitted if no break occurs at 9, the results are 9 or 10.

assume we have 2 eggs and 4 try

from what i understand, the decision tree behind the answer is that

down is break, rigth is survive
4 ----- 7 --- 9
|       |     |
1-2-3   5-6   8

the answer is 9

but if it did not break at 3 or 6 or 8, we know its 1 above it since we already tested 2 above it
so the decision tree should be

5 ----- 9 --- 12
|       |     |
1-2-3   6-7   10

the answer should be 12

im not sure if i misunderstand it or i should put it into issue