"There are certain things that we may miss, may not fully understand when we come out, but our intention is never to lie to you," he said.
Side-by-side photos of Trump's inauguration and that of President Barack Obama in 2009 show many more people on the National Mall during Obama's inauguration.
On Saturday, an angry Spicer said," This was the largest audience to ever witness an inauguration — period — both in person and around the globe."
That statement was widely taken to mean that there were more people on the National Mall to witness the inauguration than any other.
On Monday, Spicer said he did not mean to suggest that it was the case.
"I'm not" saying that, Spicer said.
"I'm saying it was the total largest audience witnessed in person and around the globe," the spokesman said.
Spicer said, "If you have up the network streaming numbers, Facebook, YouTube, all the various live streamings that we have information on so far, I don't think there's any question that it was the largest watched inauguration ever."
Spicer on Saturday also had claimed that more people had used Washington's Metro system on Friday than had during Obama's Inauguration Day in 2013. But it turned out that Spicer had compared the number of people who used the system during Trump's Inauguration Day with ridership numbers only up to 11 a.m. for Obama's day.
Ridership was lighter on Trump's Inauguration Day, according to Metro officials in Washington.
At Monday's news conference, Spicer said "the information" about ridership "came from an outside agency that we reported on."
"Knowing what we know now, we can tell a lot of those numbers are different," Spicer said. "But we were trying to provide the numbers that we had been provided."
"That wasn't like we made them up out of thin air," he said.
Spicer noted that "there are times when you guys tweet something out, or write a story, and you publish a correction."
"That doesn't mean that you were intentionally trying to deceive readers and the American people, does it?" Spicer asked. "I think we should be afforded the same opportunity."
"I'm going to come out here and tell you the facts as I know them," he said. "If we make a mistake, I'll do my best to correct it."