The Zhou Dynasty classed its vassals into five ranks, 公 侯 伯 子 男, which are usually translated into English as Duke, Marquis, Count, Viscount and Baron. The State of Lu held a rank of Marquis (侯). Accordingly, its rulers are properly referred to as Marquis of Lu (魯侯). For example, Marquis Xi of Lu (魯侯戲) whose given name was Xi.
However, within their own realms, all vassal lords may be addressed as "Duke" (公), as an honorific (because dukes are the highest of the five ranks). In addition, when a vassal lord died, he may be given a posthumous name. The title of Duke (公) was typically used for this in conjunction with one or two other words that are meant to summarise their reign.
For instance, Marquis Xi mentioned above is known as Duke Yi of Lu. The "Yi" part comes from it being a posthumous name. When Xi passed away, he was given a posthumous name to commemorate his reign: Yi (懿). Per convention, he then became Duke Yi of Lu (魯懿公).
In Chinese history, rulers who were given posthumous names are typically recorded via that name. Moreover, one of the main sources of historical knowledge on that era came from the Annals of Spring and Autumn, which was the national historical record of the State of Lu. Therefore, seeing their rulers referred to as Dukes (公) are more common than by their actual names.
This has led to them all being translated as dukes. Unfortunately, when the actual rank is lower that Duke, it became a confusing mismatch of "Marquis being hereditary dukes".