Visits different between Warehouse and OverTime sources using Report.Queue v1.4

Hi - Hoping I can get an explanation for a strange behaviour.

I'm calling a simple request via the Report.Queue API (v1.4). It looks like this.

{

                "reportDescription":{

                                "reportSuiteID":"mySite",

                                "dateFrom":"2018-01-01",

                                "dateTo":"2018-08-01",

                                "dateGranularity":"month",

                                "metrics":[

{"id":"pageviews"},

{"id":"visits"},

{"id":"uniquevisitors"}

                                ],

                              "source":"warehouse"

                }

When I run the request using "source":"warehouse", I get a number (127K) for visits for the month of January 2018. When I run the request without "source":"warehouse" - that is, as a 'standard' report, I get 155K visits for January 2018. The pageviews and unique visitor counts are the same.

Using the AA site and looking at standard metric reports, I see the 155K number. I'm not using any elements or other filters in the request.

Can anyone help me understand why the data warehouse would have different numbers?