For several reasons. For one thing, Pelosi didn't make that claim under oath, so even if she did lie, which there is little evidence of, it's not illegal. And the photo of her at a dinner with Kislyak was from years ago, and he wasn't the most senior Russian at the table. Most likely, if she recalled the meeting, she would have recalled a group dinner with Medvedev. It's much harder to believe that Sessions didn't recall any of these recent meetings over the course of this election cycle. And Sessions's responses haven't been to deny remembering the meeting. He's been trying to parse the meaning of the questions down to the bare threads. That may save him from a perjury charge, but just barely, but it sure looks like he was being less than fully honest. And then there's the context of being in an administration which has lied about and understated its contacts with Russian officials.
At bottom, it's a judgement call. But I think it's pretty definitively different.
