The issue is not whether he lied or not, the issue is whether he perjured himself our not. Perjury is when you are found to have willfully lied while under an oath to tell the truth. During his confirmation hearing, there was an exchange that went like so (between the Democratic Senator Al Franken from Minnesota and Jeff Sessions):
Franken: "CNN just published a story alleging that the intelligence community provided documents to the president-elect last week that included information that quote, ‘Russian operatives claimed to have compromising personal and financial information about Mr. Trump.’ These documents also allegedly say quote, ‘There was a continuing exchange of information during the campaign between Trump's surrogates and intermediaries for the Russian government.’
"Now, again, I'm telling you this as it's coming out, so you know. But if it's true, it's obviously extremely serious and if there is any evidence that anyone affiliated with the Trump campaign communicated with the Russian government in the course of this campaign, what will you do?"
Sessions: "Senator Franken, I'm not aware of any of those activities. I have been called a surrogate at a time or two in that campaign and I didn't have — did not have communications with the Russians, and I'm unable to comment on it."
Additionally, in Sen. Leahy submitted a written request asking whether Sessions "ever discussed the 2016 election with any Russian officials". Sessions responded with one word, "No."
While he, at the very best, mislead Congress under oath, to truly be perjury, prosecutors must prove that the defendant made a statement "willfully" that was untrue. Sessions would have had to have lied on purpose, not uttered a falsehood accidentally. His wording, whether intentional or not, left enough room to stave off a perjury charge.
Sessions recognized his error and said he should have "slowed down" and "spoken more precisely" because, according to him, he was answering the part of the question where Franken asked if he had any contact with Russian officials regarding the 2016 election specifically.
Sessions has recused himself from any investigation into Russian meddling in the US election of 2016.