There was a walled, medieval-era Japanese city in my recent game with a Samurai in it, and it had 62 strength, which seemed way too high for the period (the best unit the Japanese made at the time was a Knight). Also, I have noticed the same strength nowadays does not always translate into the same damage done by the city on its attack. Some 40-ish strength cities barely harmed my archers in the classical era (they did ca. 20 damage per shot), while cities with roughly the same strength did over 30 damage to my Crossbowmen in a later era.
Something seems buggy indeed.