Not sure which painting you are referencing, but in general it was a common artistic convention during the renaissance to paint important historical/religious/mythological figures in contemporary attire. So seeing a 12th century figure dressed in full 15th century plate in a 15th or 16th century painting isn't so strange. If the painting is from the later 16th to 18th century then it's a combination of lack of knowledge/interest in 'accurate' medieval dress, and Romanticism. If the painting is late 18th-19th century then it's Victorian Medieval Romanticism!