@loisloriot I doub't there would be anything wrong in the way power bi sums it. To troubleshoot it you will have to drill down to lower levels. Can you drill down to month's level and check with sql query on your source. The only thing I can think is why you need to use CALCULATE fuction. For your calculated column you can simply create it as, SUM([price])+SUM([tax]). See if that makes any difference.
What is the first query and how you did the modification? And what are the data type of price and tax imported in query editor? I doubt the data is not loaded correctly. Try to sort the data by price ascending and copy the data to an excel sheet in query editor. In SSMS select the data of 2015 order by price asc and then copy the data to the same sheet and compare.
Just a quick update, I did as @ankitpatira suggest. I found discrepancy for all months. I export all data for January: same number of rows, same value... except Power BI does not bother to stored useless decimal "0" :
Left SSMS export - Right PowerBI export
So I tried to change the data type in PowerBI : from Decimal Number (Auto) to Fixed Decimal Number (6), still the same wrong amount.
The sample data works in my test. I still doubt the data is not loaded correctly in your case, is there any other way to compare the data in database and in Power BI, what is the table definition(data types for price and tax) in database? By the way, except 2015's, does that measure work for other years?