I think Bally and other makers were experimenting with different connector styles, or the harness line ran out and had to make due with what they could get on short notice. You didn't stop the productions lines that were running 24/7 because of something like this. I've definitely gotten early williams games that had the IDC type and then later ones that didn't, seemingly randomly, ditto with Bally games.
IDC isn't necessarily bad, only if its been abused.