the inventory level is at 67 days (the most recent), second inventory doesn't necessary have the exact vehicle that customers wants, also it depends on which make and model that had the inventory vs affected by the earthquake.
sales decline due to:
1) 2 fewer sellings days (that is 6% of selling days, assuming 30 per month)
2) 20k less fleet sales to rental companies (comparing to may 2015), this is 10% of sales
3) they said they didn't have enough inventory for the latest release models (cruze, malibu, xt5, ct6)
4) whatever other reason you can come up with