The 1980 has had nearly twice the time to tarnish as the 1998 (I don't think anyone else mentioned this).
This isn't quite a scientific statement, but silver-plated objects tarnish faster and differently than 90% silver. Likewise, it stands to reason that 95% copper will tone/tarnish differently than a copper-plated cent.
In Memory of Crazyb0
12-26-1951 to 7-27-2020
In Memory of Tootallious
3-31-1964 to 4-15-2020