Because they only track searches made based on a search input such as “jean”, activated by either hitting the Enter key or a click on the search icon / button.
However; today most searches actually are carried out based on customers clicking the search suggestions, such as “jeans” — especially from mobile devices, where customers tend to consider search a browsing feature:
Search tracking tip:
A good tip thus is to remember to install your in page search tracking in such a manner that you track both your search field and your search suggests.
You hereby get a more accurate data set by tracking both “jean” and “jeans”.