When selecting a date range for a report, you also need to consider your job status filters.
When selecting a report with a chosen date range, vWork has to take into account that there are some job status types that may not have planned start times one the job. (for example: unassigned, or cancelled jobs may not have a start time).
If your report filters include all job statuses which is the default report setting, then you quite often can get many more jobs than you expect...
A good rule to follow is to start with only reporting on completed jobs, as these will always have a start time.
Of course if you are looking for data on a specific state like started jobs, this should work as normal.