New Column Mysterious Behaviour

I've made this query in Qmf for Windows (using DB2)

I select these columns:

TABLEONE: SEC
TABLETWO: B_DATE and E_DATE (they are numbers – not dates); RATE (is a number)

I first calculate the difference between E_DATE and B_DATE and between a chosen date and B_DATE
creating the new columns GG_CED_C and GG_MAT_C.
These new columns are calculated perfectly (they make the difference
between the two dates according to the 30/360 date count convention) and don’t give me any problems. They are all filled with numbers.
I’m able to use these new columns to make other calculations. For example I succeed in calculating
RATE/GG_CED_C* GG_MAT_C
but if I try to calculate
RATE/360*GG_MAT_C AS RATEO
the result is that in some records I have my new column RATEO correctly calculated, while in some other records there is no value in the column.

For example:

B_DATE E_DATE RATE GG_CED_C GG_MAT_C RATEO
20061229 20070630 3,995 180 90 0,99875
20061229 20070630 4,185 180 90

Dates are identical and so are GG_CED_C and GG_MAT_C: why don't
I have the result in the second record????

SELECT

SEC, B_DATE, E_DATE, RATE, GG_CED_C, GG_MAT_C,
RATE/ 360 * GG_MAT_C AS RATEO

FROM (
SEC, B_DATE, E_DATE, RATE, GG_CED_C, GG_MAT_C

(YEAR2 - YEAR1)*360 + (MONTH2 - MONTH1)*30 + (DAY2 - DAY1) AS GG_CED_C,
(YEARRIL - YEAR1)*360 + (MONTHRIL - MONTH1)*30 + (DAYRIL - DAY1) AS GG_MAT_C,
FROM (
SELECT
A.SEC, , B_DATE, E_DATE, RATE,
TRUNC(B_DATE/10000,0) AS YEAR1 ,
TRUNC(MOD(B_DATE,10000)/100,0) AS MONTH1 ,
CASE
WHEN MOD(B_DATE,1000)= 229 THEN 30
WHEN MOD(B_DATE,1000)= 228
AND MOD(TRUNC(B_DATE/10000,0),4) > 0 THEN 30
ELSE MIN(MOD(B_DATE,100),30)
END AS DAY1,
TRUNC(E_DATE/10000,0) AS YEAR2 ,
TRUNC(MOD(E_DATE,10000)/100,0) AS MONTH2 ,
CASE
WHEN MOD(E_DATE,1000)= 229 THEN 30
WHEN MOD(E_DATE,1000)= 228
AND MOD(TRUNC(E_DATE/10000,0),4) > 0 THEN 30
ELSE MIN(MOD(E_DATE,100),30)
END AS DAY2,
TRUNC(&DRIL/10000,0) AS YEARRIL ,
TRUNC(MOD(&DRIL,10000)/100,0) AS MONTHRIL ,
CASE
WHEN MOD(&DRIL,1000)= 229 THEN 30
WHEN MOD(&DRIL,1000)= 228
AND MOD(TRUNC(&DRIL/10000,0),4) > 0 THEN 30
ELSE MIN(MOD(&DRIL,100),30)
END AS DAYRIL,

FROM TABLEONE A, TABLETWO C

What's happening?
After what I've seen in this query I'm wondering if SQL is reliable
when it comes to calculations (after all its main duty is to query data and not to make calculations among them)(?????.

Please give an explanation to this mystery.

Thank you.

Anna - Verona (Italy)