The GL Distribution always overwrite the Activity dimension. Please look as a scenario under this forum topic http://red1.org/adempiere/viewtopic.php?f=31&t=1561
Windows 7 & Server 2003, PostgreSQL 8.4/9/1
Sorry for the generalization - this problem happens only when Payroll process is posted. Because I retested it when Posting is done from GL Journal & invoices -vendor, it works as expected.