"The Japanese had a number of incentives in place to keep their buyers in their vehicles," Johnson told analysts and reporters during a conference call. "I think that's probably giving them a bit of a bump this month."
'Dose of reality'
In a note to investors, Morgan Stanley analyst Adam Jonas said GM's "negative surprise" on October sales volume "is a gentle dose of reality that GM must give back some of the share it had borrowed from competitors."
Through September, GM had gained one percentage point of U.S. market share, to 20 percent from 19 percent, while Japanese rivals struggled to overcome production shortages stemming from the March earthquake. Toyota's share slid 2.7 points to 12.5 percent. Honda's share dropped 1.6 points to 9.0 percent.
Japanese automakers offered larger incentives in October than they had a year earlier, according to data compiled by TrueCar.com. Nissan's incentive spending rose 15 percent, while Honda's was up 18 percent and Toyota's rose 13 percent. GM's incentive spending rose 3 percent.
Sales of the hot-selling Chevrolet Cruze slowed last month to 14,295 units, the lowest level since January. For five straight months through September, the Cruze had been the top-selling U.S. compact car, as Toyota Corolla and Honda Civic sales sagged.
Johnson said the drop in Cruze sales from summer levels was largely due to seasonal factors. He said the sales pace for Cruze, when factoring in seasonal adjustments, was up slightly from September.
GM also saw inventories rise during the quarter, from 558,157 units at the end of September to 592,151 on Oct. 31. That boosted its supply from 67 days to 82, the automaker said.
Jonas said that the higher inventory levels "may rehash questions about production schedule risk and incentives."