calculating the total number of rows in a table - fails when it is a heap - is there any fix?
make use of DBCC UPDATEUSAGE
as mentioned in the comments by  Dan Guzman
has produced the following results:

DBCC UPDATEUSAGE(N'WMSData',N'dbo.WMS_SLALoad') WITH COUNT_ROWS




After that the results of my original select was accurate enough for my purposes.