It sounds to me like you are interpreting last index as previous index where it's actually the index of the last occurrence (or perhaps final occurrence) of that substring within the source string. If the substring's last position starts at n then as long as the fromIndex position is between 0 and n inclusive, you will return n.

Based on the source found here, it appears as if lastIndexOf does traverse from the end of the string, towards the beginning. An excerpt is posted below for convenience; note the decrement operations as it is keeping track of i and j to find the last match.

startSearchForLastChar: while (true) {
    while (i >= min && source[i] != strLastChar) {
        i--;
    }
    if (i < min) {
        return -1;
    }
    int j = i - 1;
    int start = j - (targetCount - 1);
    int k = strLastIndex - 1;

    while (j > start) {
        if (source[j--] != target[k--]) {
            i--;
            continue startSearchForLastChar;
        }
    }
    return start - sourceOffset + 1;
}