Aashpreet, 

the French is the original phrase and it is correct. 

Of course, if they changed multiple cars every year  ‘ nous changions de voitures ‘ would have been used. It is a general statement, not about ‘ specific/specified ‘ cars. 

The original statement is normally assumed to be what the speaker/writer meant, although occasional ‘errors’ can happen . Any significant discordance between original and translation requires a change in the translation, not the original expression.

However, there is more than one way to translate the French sentence to English, and the translation given is one of these.

At least in my part of the world, it is quite colloquial in English to use the plural in the context here eg ‘ he changed cars/jobs every year’. 

From wordreference link below under ‘changer de..’ you will see that there are 2 similar examples listed 

 https://www.wordreference.com/fren/changer 

changer de [qch]vtr ind(remplacer)change⇒ vtr  switch⇒ vtr Dans cette société, on change de poste tous les 5 ans. Je pense changer d'opérateur téléphonique. In this company, we change jobs every five years. I'm thinking of switching phone companies.