Because 1234567980 (your solution) is not the next bigger number after 1234567890 (the input) ???

You can write them down:

1234567890 # input
1234567891 # +1
1234567892 # +2
...
1234567900 # +10
1234567901 # +11
...
1234567908 # +18, AND THE FIRST TIME WE ENCOUNTER A BIGGER NUMBER WITH THE SAME DIGITS AS THE INPUT VALUE
...
1234567978 # +88
1234567979 # +89
1234567980 # <- your solution, which does have same digits as input, BUT is bigger than 1234567908 --^