Wrong count of rows on different time ranges

I have the following query:

sum(count_over_time({server="prod", operation="Profil aufgerufen"} |= `profileType=basic` | logfmt | __error__=`` [$__interval]))

Selecting a greater time range gives me less result rows.
Last 2 days: 540 rows
Last 7 days: 310 rows

How is this possible? Am I missing something or is the query wrong?