How are reserved and formal synonyms?

In the dictionary reserved means - not revealing emotions and formal means - prim or stiff. I saw they were synonym in the Google dictionary. What makes these two words similar, I wonder?

Solution 1:

Reserved means "tending to keep your feelings or thoughts private rather than showing them," while formal can mean "correct or conservative in style, and appropriate for official or serious situations or occasions." The words are related in that someone who is formal by nature or upbringing will also likely tend to be reserved. They are not "true synonyms," however, in that they are not interchangeable but are related in terms of manners and comportment. See the linked definition of synonym at merriam-webster.com.