There are 3,142 counties and county equivalents across the fifty states and the District of Columbia. There are 435 congressional districts. Neither number divides into the other and neither boundary respects the other.
Of the 435 districts, 79 sit entirely inside a single county. Most of the remaining 356 cross at least one county line, and many counties are themselves split between several districts. Los Angeles County contains ten whole districts and parts of others. Cherry County, Nebraska, is one piece of a district that has 78 more.
Why this shows up on your television
Votes are administered by counties, so counties are what report. Every results feed on election night is a county feed, and the county-shaded map is the map you see because it is the map that exists at 9pm. House control is not decided in any of those units.
For a state like Alabama, where districts are large and counties nest reasonably well, the county map is a decent proxy. For a state like California, where 21 of 52 seats never leave their home county, the county map cannot resolve the race at all. Shading Los Angeles County one colour hides ten separate contests.
A footnote on that 3,142
The Census Bureau's official count is 3,143. The boundary file this site ships says 3,142, and the missing one is Connecticut: the file predates June 2022, so it carries Connecticut's eight legacy counties instead of the nine planning regions that replaced them. Nine minus eight is the entire discrepancy.
That is written up properly in the corrections note. It is mentioned here because a number being off by one is usually a rounding artefact and this one is a boundary change nobody outside Connecticut noticed.