Let's recap...Your confusing me now
I don't understand, why was he not found guilty.
It is not (per se) an offence to lie. If you take an oath in court, then yes, you have committed an offence.
Boris lied because he knew the figure he kept on repeating was false. However politicians put information (true or false) into the public domain to be debated. It can not be construed as "misleading the public in a public office" because everyone has the right to decide the merits of the information based upon what they believe to be true individually.