Using character type for MODEL dimension causes rules using CV() to repeat results - why ?

I am using MODEL clause to calculate balance and compound interest.
Using generated date converted to YYYYMM as key to dimension I get balance always repeated each 12 months.  
Can someone explain why ?

By using the generated seqence number  directly as the dimension-key fixed the problem.  


Simplified sql of both cases:
--Here balance reapeats every 12 months
SELECT * FROM (
  SELECT    TO_CHAR( ADD_MONTHS(DATE '2000-01-01',  SEQNR),'YYYYMM') AS YEARMON, 1 AS BASE , 0 AS BAL FROM (select LEVEL-1 AS SEQNR from dual  connect by level <= 100)
ORDER BY YEARMON
)
MODEL
dimension by (YEARMON)
measures (BASE, BAL)
RULES (
      BAL[ANY] ORDER BY YEARMON =   nvl(BASE[CV()],0) +   nvl(BAL[CV()-1],0)
)  ORDER BY YEARMON


--Here balance get increased until lastrow (as disired)
SELECT * FROM (
  SELECT   SEQNR,  TO_CHAR( ADD_MONTHS(DATE '2000-01-01',  SEQNR),'YYYYMM') AS YEARMON, 1 AS BASE , 0 AS BAL FROM (select LEVEL-1 AS SEQNR from dual  connect by level <= 100)
ORDER BY SEQNR
)
MODEL
dimension by (SEQNR)
measures (YEARMON,BASE, BAL)
RULES (
      BAL[ANY] ORDER BY SEQNR =   nvl(BASE[CV()],0) +   nvl(BAL[CV()-1],0)
)  ORDER BY YEARMON