(Note @all, links are now both redirecting properly)
Notice the problem description carefully:
“on each path the next number is located on the row below, more precisely either directly below or below and one place to the right;”
The highlighted statement should be fully understood in order you can understand problem.
Looking at the example that it was provided to you:
5 4 3
On the 1st row you obviously choose 1, as it’s the only possible number.
Now, on 2nd row you can choose a number that it’s either below or to the right of 1 (on this case, we can choose either number 2 or number 1 as we have only two numbers.)
Two things can happen:
We choose number 2:
If this is the case, then on row 3, we can only choose numbers 5 or 4 as they are respectively below, or one place to the right of number 2 located on previous row;
Same procedure applied on number 1 of row 2, leads to only being able to choose elements 4 or 3 on row 3.
By following this rules we see max sum that is possible is actually 8 and not 11.