"Why is it that most 2100s-era human warships in this game of yours are placed in the same battle rating as ships built 20 or even 40 years later than them?"
"They built them with much more 'dakka' than most at their time. If there were historical battle ratings then much of low and mid tier would be almost unplayable because their ships were just that good for their time period."
Examples:
The destroyer Hatsukaze is at the same battle rating as the Antarean destroyer Aukani due to her armament of five 25-inch railguns, despite the latter having been commissioned several decades later.
The first dreadnought battleship in the galaxy, UNS Dreadnought, is at the same battle rating as AIS Omikron. Omikron was commissioned almost two decades later.
And humanity's first fleet carrier, Wright, is at the same battle rating as the Antarean fleet carrier/battleship conversion Asgtia, which was commissioned almost a decade later.