Union Changing Column Results?

I have a problem with a union all that seems to be changing the results of a column based on using the union, when I run the querys separatly I get the expected results in the field!

Basically 3 queries pulled together for reporting purposes.

Orders (which can include a gift so the gifts are separated in the second query).
OrderGifts (pulled out of the orders).
Gifts (straight gifts with no associated orders).

The problem is showing up in the first ORDERS query. I can pull a specific order and the calculation for the AMOUNT(eg:$15.20) is correct, but when I combine the tables I get a different AMOUNT(eg:$15.199999999999999).

Here are the values for the fields in the amount calc:
totitems=12.9500
totcredits=.0000
discount=.0000
tax=.0000
refund=.0000
convrate=1.0
postage=2.2500
giftamt=15.0000


I tried converting each of the fields to an decimal befre the calc and same results

I tried ROUND and same thing!

I have been trying to chase this down all day, cant figure out what the problem is or how to get around it...whats going on here I am missing?


Here is the query, if you need any other info just let me know, I would GREATLY appreciate ANY direction of figuring this out!


select * from
(
--orders
select
acctnbr as PartnerId,
'O' as TrnType,
ordnbr as TrnId,
0 as FundId,
odate as WorkDate,
adate as PostDate,
sdate as ShipDate,
cast(
(
IsNull(totitems, 0) -
IsNull(totcredits, 0) -
IsNull(discount, 0) -
IsNull(tax, 0) -
IsNull(refund, 0) +
IsNull(postage, 0)
) * IsNull(convrate, 1)
as Decimal(10,2))
as Amount,
batchnbr as BatchId,
motvcode as MotivationCode,
currcode as CurrencyType,
Status,
paycode as PaymentType,
paytype as CardType,
paynbr as CardNbr,
expire as CardExpire,
ChargeAuth,
ConvRate,
MediaCode,
hcf.dbo.KmaiProjMotvTest(0,motvcode) as ProjMotvName
from kmai.dbo.o01omst

UNION all

--orders gifts
select
acctnbr as PartnerId,
'G' as TrnType,
giftref as TrnId,
giftfundid as FundId,
odate as WorkDate,
null as PostDate,
null as ShipDate,
ROUND((giftamt * IsNull(convrate, 1)),2) as Amount,
batchnbr as BatchId,
giftmotvcode as MotivationCode,
currcode as CurrencyType,
Status,
paycode as PaymentType,
paytype as CardType,
paynbr as CardNbr,
expire as CardExpire,
ChargeAuth,
ConvRate,
MediaCode,
hcf.dbo.KmaiProjMotvTest(giftfundid,giftmotvcode) as ProjMotvName
from kmai.dbo.o01omst
where adate is null
and giftfundid <> ''

UNION ALL
--gifts
select
acctnbr as PartnerId,
'G' as TrnType,
gh.giftref as TrnId,
convert(int, fundid) as FundId,
gdate as WorkDate,
applydate as PostDate,
null as ShipDate,
ROUND((IsNull(gd.amt, 0) * IsNull(convrate, 1)),2) AS Amount,
batchnbr as BatchId,
motvcode as MotivationCode,
currcode as CurrencyType,
Status,
paycode as PaymentType,
paytype as CardType,
paynbr as CardNbr,
expire as CardExpire,
ChargeAuth,
ConvRate,
MediaCode,
hcf.dbo.KmaiProjMotvTest(fundid,motvcode) as ProjMotvName
from kmai.dbo.g03ghdr gh
inner join kmai..g04gdtl gd
on gh.giftref = gd.giftref

)
UnionOfGiftsAndOrders