  There is a broad consensus that news media outlets incorporate ideological
biases in their news articles. However, prior studies on measuring the
discrepancies among media outlets and further dissecting the origins of
thematic differences suffer from small sample sizes and limited scope and
granularity. In this study, we use a large dataset of 1.8 million news
headlines from major U.S. media outlets spanning from 2014 to 2022 to
thoroughly track and dissect the fine-grained thematic discrepancy in U.S. news
media. We employ multiple correspondence analysis (MCA) to quantify the
fine-grained thematic discrepancy related to four prominent topics - domestic
politics, economic issues, social issues, and foreign affairs in order to
derive a more holistic analysis. Additionally, we compare the most frequent
$n$-grams in media headlines to provide further qualitative insights into our
analysis. Our findings indicate that on domestic politics and social issues,
the discrepancy can be attributed to a certain degree of media bias. Meanwhile,
the discrepancy in reporting foreign affairs is largely attributed to the
diversity in individual journalistic styles. Finally, U.S. media outlets show
consistency and high similarity in their coverage of economic issues.
