It's not more precise, it becomes inaccurate.

A man says he's 6'6". Sure. If he's anywhere between 6'5½" and 6'6½", that's true.

You say he's 198.12cm tall. The range of this being true is now thinner than a needle. It has gone far beyond what anyone actually measures. In over 99% of cases, it's not true, and if it is, it won't be for long, because the human body isn't nearly that consistent from breath to breath.

The conversion with spurious false precision has made the number go from true to not true.

The man is six foot six, yes, true. The man is 198.12cm - no he isn't.