Last Six Months of Sales

Transact-SQL
2

I don't think so and there are two reasons for that:

  1. invoice no 1622 has invoice date way earlier than 2016-05-29 minus 6 months
  2. you are using "with (nolock)" which can give you all kinds of results (see here)

One way of solving your issue, could be:

select b.faknr as InvoiceNr
      ,b.fakdat as InvoiceDate
      ,datepart(year,b.fakdat) as YearNr
      ,datepart(month,b.fakdat) as MonthNr
      ,b.dagbknr as InvoiceType
      ,b.debnr as DebtorNr
      ,b.inv_debtor_name as DebtorName
  from (select debnr
              ,dateadd(month,-6,max(fakdat)) as fakdat
          from dbo.frhkrg
         group by debnr
       ) as a
       inner join dbo.frhkrg as b
               on b.debnr=a.debnr
              and b.fakdat>=a.fakdat
;