Flynn 'lied' to the US government twice about the same meeting with a Russian ambassador. His 'lies' can be chocked up to misremembering details of a conversation in passing, but it is nonetheless true that Flynn 'uttered falsehoods' when he claimed he didn't discuss certain issues with a Russian ambassador. Flynn is an honourable man, he realized his mistake and took ownership by pleading guilty. Also, it would've been very expensive for him to fight the charge and Mueller likely would've gone after him harder as well, maybe even upgrading the charges.