inconsistent result compare to other lib

[fzxu]

Having a big integer, this is the string representation (10 radix): "61112663125671126633722436054102056730607837166180410453588415302234407842306"

The method toDERInteger() which convert to the bigendian bytes would return:
[0, -121, 28, -118, -93, -127, -101, 97, 127, 69, -4, -15, 127, -126, 42, -104, -89, 8, 44, 66, 30, 73, -84, -5, -54, -93, -87, 83, -28, 113, 30, 50, 2]

But the same method from pointycastle returns:
https://github.com/PointyCastle/pointycastle/blob/master/lib/src/utils.dart#L21

unsigned:
[135, 28, 138, 163, 129, 155, 97, 127, 69, 252, 241, 127, 130, 42, 152, 167, 8, 44, 66, 30, 73, 172, 251, 202, 163, 169, 83, 228, 113, 30, 50, 2]

convert to signed:
[-121, 28, -118, -93, -127, -101, 97, 127, 69, -4, -15, 127, -126, 42, -104, -89, 8, 44, 66, 30, 73, -84, -5, -54, -93, -87, 83, -28, 113, 30, 50, 2]

Wondering what is the leading 0...