Why is it that sometimes when I do a search I can't find the item in the results?

For instance I searched for the the phrase "Nantwich Union" in 1825 and got this result:
https://www.britishnewspaperarchive.co.uk/search/results/1820-01-01/1829-12-31?basicsearch=%22nantwich%20union%22%20&phrasesearch=nantwich%20union&exactsearch=false&retrievecountrycounts=false&sortorder=score

but the term Nantwich Union doesn't appear in the result:
https://www.britishnewspaperarchive.co.uk/viewer/bl/0000389/18250614/005/0001