Generally when I come across this it is usually timezones, whitelisted indexes and/or default indexes configured for the user's roles. Since you said it is not time zones I would check the user's role allowed indexes and (if the index is not specified in the search) what are the default searched indexes for their role. If no luck there you can take a deeper look at the user's role search restrictions and see if there is anything there that may be the cause of the discrepancy.
