中国15名副省级市委书记中，已有七名“70后”。
Seven of China's 15 deputy-provincial-level municipal party secretaries were born in the 1970s.
这两句话确实存在逻辑重心的错位。

中文原句的核心是“已有七名”，强调“现状”和“数量”；而英文译句的时态（were）和语序，却把重心放在了“出生年份”这一“事实属性”上。

最地道、外媒常用版本

Seven of China's 15 sub-provincial city party secretaries are post-70s.

更正式、严谨版本

Of the 15 party secretaries of sub-provincial cities in China, seven were born in the 1970s.

简洁新闻体

Seven out of 15 sub-provincial city party secretaries in China are from the post-1970s generation.