card: explain the row count the sidebar reports

README.md

@@ -109,6 +109,11 @@ reservoirs, the night markets.
 The successor to [`lianghsun/Formosa-bench`](https://huggingface.co/datasets/lianghsun/Formosa-bench)
 (349 items, four categories).
 
+> **The row count on this page.** Hugging Face reports a dataset's row count as
+> the sum over every config, and `all` covers the same questions as the 13
+> category configs. The sidebar therefore reads 3,392 where the benchmark holds
+> **1,696 questions**, each counted twice.
+
 ## Overview
 
 | Property | Value |