PREVIOUSMONTH

You have to read the fine print here to understand how PREVIOUSMONTH (and similar functions) work:

https://support.office.com/en-US/article/PREVIOUSMONTH-Function-DAX-72fb8dda-6cd6-49e6-b9b4-8a5f632439c2

"This function returns all dates from the previous month, using the first date in the column used as input. For example, if the first date in the dates argument refers to June 10, 2009, this function returns all dates for the month of May, 2009."

So, mainly you have to use this in a "context aware way". Here is the example from the page:

Example

The following sample formula creates a calculated field that calculates the 'previous month sales' for the Internet sales.

To see how this works, create a PivotTable and add the fields, CalendarYear and MonthNumberOfYear, to the Row Labels area of the PivotTable. Then add a calculated field, named Previous Month Sales, using the formula defined in the code section, to the Values area of the PivotTable.

=CALCULATE(SUM(InternetSales_USD[SalesAmount_USD]), PREVIOUSMONTH('DateTime'[DateKey]))

My understanding of how this works is that the row labels are context filters that filter the 'DateTime' table to that specific year and month, so in the context of the pivot table (matrix) for each particular row, the 'DateTime' table has been filtered such that when you pass PREVIOUSMONTH the 'DateTime'[DateKey] column, you are only passing in the dates for a specific year and month, like June 2009. Thus, PREVIOUSMONTH sees that and goes and grabs the dates for the previous month and passes those back as filters to the CALCULATE function. I don't think that the page really explains it all very well, but that is my understanding of how it is supposed to work.

Here is a good article on context in DAX formulas that might help as well.

https://support.office.com/en-US/article/Context-in-DAX-Formulas-2728fae0-8309-45b6-9d32-1d600440a7ad