They are totally different in meaning. Google them and see. What gave you the impression that they were synonyms?

They are completely different. Since Su.Ki. wants you to look them up I won't give the meaning, but I will give some examples of use. "The nasal spray flu vaccine actually contains live virus, but it has been attenuated. Because the virus is attenuated, it doesn't cause flu." "This fiber optics cable has an attenuation rate of 0.2 dB/km. When a signal travels 15 kilometers through this cable, it loses about half of its strength." "I'm guilty of using bad language, but there are extenuating circumstances: my computer just crashed again." "You're late again. Can you say anything in extenuation?"