Variable Results Using Totals

OK, this is a stumper.

I'm trying to determine if an employee changed departments in 2007.

We maintain a table that records every time an employee transfers, giving an effective date of the change. But, this table also records transferring to a different shift without a department change.

So, my logic was to look at what an employee's most recent assignment was as of 12/31/06 - this way, even if the employee's last transfer was June 3, 1827, it would still be picked up.

Then, I would see if there was any department change in 2007 by comparing any records recorded in this table in 2007 with the list from 2006 and pulling out those records that had different departments.

My first attempt was to use a query for 2006 and earlier with totals, grouping on the employee ID and grabbing the Max Effective date along with the department ID (org). Like so;

SELECT dbo_assignment_log.ppms_ID, Max(dbo_assignment_log.effectivedate) AS MaxOfeffectivedate, dbo_assignment_log.workingorg
FROM dbo_assignment_log
GROUP BY dbo_assignment_log.ppms_ID, dbo_assignment_log.workingorg
HAVING (((Max(dbo_assignment_log.effectivedate))<=#12/31/2006#))
ORDER BY dbo_assignment_log.ppms_ID, Max(dbo_assignment_log.effectivedate);

The result looked like this;

ppms_IDMaxOfeffectivedateworkingorg
289874/12/20022116
2898711/29/20062121
289885/31/19952116
289888/7/19962124
289896/1/20042114
2898911/22/20052176
289901/25/20022116
2899012/18/20022122
2899112/4/19922124
2899111/26/19972122
289919/27/20002155
289929/23/19932128
289937/27/20052134

All well and good. But, then I decided that I didn't want the org in the list since it was giving me extra records (because it was on Group By), so I took it out and ran the query again like this;

SELECT dbo_assignment_log.ppms_ID, Max(dbo_assignment_log.effectivedate) AS MaxOfeffectivedate
FROM dbo_assignment_log
GROUP BY dbo_assignment_log.ppms_ID
HAVING (((Max(dbo_assignment_log.effectivedate))<=#12/31/2006#))
ORDER BY dbo_assignment_log.ppms_ID, Max(dbo_assignment_log.effectivedate);


Much to my surprise, some records were lost.

ppms_IDMaxOfeffectivedate
2898911/22/2005
2899012/18/2002
289919/27/2000
289929/23/1993
289937/27/2005
289948/4/2004
289985/18/2005
289999/29/1999
290006/7/2000
290022/4/2005
290038/24/2005
290045/8/2002
290096/29/2005

What might cause it to drop, for example, the entries for IDs 28987 and 28988?

There are a lot of other fields in the table, but, as far as I know, they shouldn't matter.

Any ideas?