access qry select vs update problem

[AndyBell]

I have been working on this access in access as a novice for several years
and on this db for several weeks [this is my third database.] I have found
great help in the newsgroups using google - thanks

However I am stumped on this one - Some background
I am using dmax to eliminate multiple payments from a single payee in one
month - retaining just the payment info for the most recent month.

The way the db is set up is to draw payment info from a Quickbooks report
and then converts some of the fields [txt to date, etc] and pastes the "new"
payment information in with the "old" name and address info stored in a
table called "tblBasis"

This takes place in two steps the first draws and converts the data from
linked excel file [the QB report called "tblQBmonthlyExport"] and appends to
a table called "tblInterimMonthly" - - this works fine.

However when I want to update the old data [tblBasis] and eliminate
duplicate payments during one month [keeping the one for the most recent
month due] is where I am having problems.

A query like the following returns the correct number of records 36 [there
are 39 payments this particular month for 36 accounts]

SELECT tblInterimMonthly.DateOccurance, tblInterimMonthly.BalanceDue,
tblInterimMonthly.DateLastPayment, tblInterimMonthly.CustAcctNum FROM
tblInterimMonthly
WHERE
(((tblInterimMonthly.DateOccurance)=DMax("DateOccurance","tblInterimMonthly","[CustAcctNum]
= " & [tblInterimMonthly].[CustAcctNum])));

However when I convert into an update qry it returns 37 records [and I can
see one duplicate]
UPDATE tblInterimMonthly INNER JOIN TblBasis ON
tblInterimMonthly.CustAcctNum = TblBasis.CustAcctNum SET
TblBasis.DateOccurance = tblInterimMonthly.DateOccurance,
TblBasis.BalanceDue = tblInterimMonthly.balacedue, TblBasis.DateLastPayment
= tblInterimMonthly.datelastpayment
WHERE
(((TblBasis.DateOccurance)=DMax("DateOccurance","tblInterimMonthly","[CustAcctNum]
= " & [tblInterimMonthly].[CustAcctNum])));

I tried it in a different way [two steps using the first qry called
'qry2DMAXw_36Records" which produces the correct records] and it returns 35
records
UPDATE qry2DMAXw_36Records INNER JOIN TblBasis ON
qry2DMAXw_36Records.CustAcctNum=TblBasis.CustAcctNum SET
TblBasis.DateOccurance = qry2DMAXw_36Records.dateoccurance,
TblBasis.BalanceDue = qry2DMAXw_36Records.balancedue,
TblBasis.DateLastPayment = qry2DMAXw_36Records.datelastpayment;

Yes the qry names are strange but I have done and redone this several ways
over the last few days and am stuck. Some of the syntax is not the most
readable [ I tend to use the access qry authoring view ]

Any help would be appreciated

I have posted a copy of the db here
http://www.habilee.org/CRinfo.htm there is a link at the bottom of the page
to a zip file it is about 200k so you can see the data, perhaps there is a
data peculiarity that is causing this that I am not seeing. Unzip it into
folder called c:\AAHACreditReporting [for some of items not necessarily
related to this] for it all to work.
the queries in discussion on the db are named
qry2DMAXw_36Records
qryUpdate_nostep_returns37
qryUpdate_step_returns35\

FYI this db is to allow the 37 Habitat for Humanity affiliates in Alabama to
work together and report payment history of our home owners. This db will
be at each location and a complierdb at our Association office -so some
items are automated for simplicity but others to allow for different
bookkeeping systems.
Other input appreciated as well.