Join or Where?

Hi,
could anybody tell me what can be the difference between these queries?
There are a big difference but I can't understand the reasons. For some
reasons the firs query calculates getup column in a wrong way. It allways
devides with 10. (???) While the getupquantity column comes well.

With the second query everything is ok.

Thank you,

-- Csaba

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select (select getupquantity from t_products where id=productid) as
getupquantity,
       quantity,
       (select quantity/getupquantity) as getup
from t_stockchanges, t_products
where (getupquantity<>0)
limit 30;

RESULT:
 getupquantity | quantity | getup
---------------+----------+-------
            10 |      100 |    10
            10 |    10600 |  1060
            15 |    15150 |  1515
            13 |    13650 |  1365
          17.5 |    17500 |  1750
          5.75 |     5750 |   575
           2.5 |     2500 |   250
           1.5 |     1500 |   150
            16 |    16000 |  1600
             2 |     2000 |   200
           5.5 |     5500 |   550
           4.5 |     4500 |   450
             2 |     2000 |   200
----------------------------------------------------------------------------
-------
select t_products.getupquantity as getupquantity,
       quantity,
       (select t_stockchanges.quantity/t_products.getupquantity) as getup
from t_stockchanges
join t_products on (t_products.id=productid)
where (getupquantity<>0)
limit 30;

RESULT:
getupquantity | quantity | getup
---------------+----------+-------
            10 |      100 |    10
            10 |    10600 |  1060
            15 |    15150 |  1010
            13 |    13650 |  1050
          17.5 |    17500 |  1000
          5.75 |     5750 |  1000
           2.5 |     2500 |  1000
           1.5 |     1500 |  1000
            16 |    16000 |  1000
             2 |     2000 |  1000
           5.5 |     5500 |  1000
           4.5 |     4500 |  1000
             2 |     2000 |  1000
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