Help!! A question to answer 2.2

[ItsBean]

The answer tells me that the grammer is ambiguous by giving the example below, but the example confused me.I don't know why both trees represent the same string .

[magneto0124]

Look at the leaves, the upper one has abεaεbε = abab, the lower one has aεbaεbε = abab, so they are the same strings.

[melvas6]

I'm very much confused with this. The strings are same. But aren't both the trees same?