Status and resolution are two separate things.
Your query, translated into English, is "issues that have a resolution date before the start of the month".
It has nothing to do with "status = resolved" directly - it's looking at the date that a resolution was set on the issue. Which may or may not be the date the issue was moved into "resolved"
The Status Resolved is not the same thing than the Date Field Resolved (also known as resolutiondate).
You can set your workflow so that, on entering Resolved status, a resolution is automatically set (or, alternatively, popup a dialog to set a resolution).
As soon as an issue has a value on the field Resolution, the date in which it happened will be recorded in the resolved/resolutiondate field.
It is likely that the issue lacks a resolution date. Try this JQL query to check it:
status = Resolved AND resolution is EMPTY
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