First thing to note is that it depends what data you are comparing. For example in Query Data if you run order item sales report and add up all the item costs, it won't include things like postage etc. Financial Summary should give you plain and simple for all orders which are Paid and not Cancelled and not on hold, so if you are interested in straight forward Revenue (including shipping, tax etc) that the place to go.
The reason why you see 31st March in the breakdown is indeed strange, I have a feeling it has something to do with UTC offset in the date picker as I can see its actually requesting data for startDate: 2019-03-31T23:00:00.000Z endDate: 2019-04-30T22:59:59.999Z, which is fine, but why does it include the whole of 31st March is a mystery. I will get back to you on this one.
ok, found a problem on Financial Summary screen with the datepicker on the UI. Looks like it adjusts your date selection to account for your timezone (which is UTC +1) and includes the whole of previous day.
Any questions, doubts about the numbers, let me know I can check and compare.
The issue is now fixed.
Can anyone please advise me on where to retrieve the most accurate sales figure? I have gone to Financial Summary. Query Data and processed orders and they all give me a different figure. Also I ran a report for April for a financial summary and when i looked at the breakdown it included the 31st March!
I would appriciate any help, thank you!