https://github.com/nmigen/nmigen-boards/blob/68cbc9a614820914249d34f25cd5cfb2c6f1c584/nmigen_boards/ulx3s.py#L113-L129
After the first 10 diff pairs, someone forgot to finish their copy-paste, and alongside that not all the diff pairs have an associated resource.
https://github.com/nmigen/nmigen-boards/blob/68cbc9a614820914249d34f25cd5cfb2c6f1c584/nmigen_boards/ulx3s.py#L101-L106