[1] Top 25% means that this radical represents 25% of Jōyō kanji.
[2] Many radicals are not commonly written by themselves so people wouldn't know the technical hiragana reading given here.
The simplified table of Japanese kanji radicals page only lists common readings.
The 214 Kangxi radicals are technically classifiers as they are not always etymologically correct,[3] but since linguistics uses that word in the sense of "classifying" nouns (such as in counter words) dictionaries commonly call the kanji components radicals.
As dictionaries have moved from textbooks to interactive screens the term "radicals" seems to now be used for any kanji component used in a visual search.