MDX date from to : issue

Hi all,

Now I have one weried issue:

WITH
MEMBER [ACTV DATE].[YEAR].[YTD] AS
AGGREGATE(STRTOMEMBER('[ACTV DATE].[Month].&[2013-01-01T00:00:00]'):STRTOMEMBER('[ACTV DATE].[Month].&[2013-02-01T00:00:00]'))

MEMBER [MEASURES].[ACTIVATIONS_RATIO] AS ([MEASURES].[FACT CONTRACT COUNT],[ACTV DATE].[YEAR].[YTD])/
([MEASURES].[FACT CONTRACT COUNT])

Member  [MEASURES].[ACTIVATIONS_Count] as  ([MEASURES].[FACT CONTRACT COUNT],[ACTV DATE].[YEAR].[YTD])

SELECT {
[MEASURES].[ACTIVATIONS_Count]
,
[MEASURES].[FACT CONTRACT COUNT]
,[MEASURES].[ACTIVATIONS_RATIO]
} ON COLUMNS

FROM [Sales]
WHERE
(STRTOMEMBER('[CALCULATE DATE].[PK Date].&[2013-02-28T00:00:00]')
,STRTOMEMBER('[APPL DATE].[PK Date].&[2013-01-01T00:00:00]'):STRTOMEMBER('[APPL DATE].[PK Date].&[2013-01-31T00:00:00]')
--,STRTOMEMBER('[APPL DATE].[CALENDAR].[Month].&[2013-01-01T00:00:00]')
--, STRTOMEMBER('[APPL DATE].[Month].&[2013-01-01T00:00:00]'):STRTOMEMBER('[APPL DATE].[CALENDAR].[Month].&[2013-01-01T00:00:00]')
)

I get the result like below, this result is correct:

ACTIVATIONS_Count FACT CONTRACT Count ACTIVATIONS_RATIO
11283                     16299                        0.692251058347138

When I use STRTOMEMBER('[APPL DATE].[CALENDAR].[Month].&[2013-01-01T00:00:00]')

I get the double result

ACTIVATIONS_Count  FACT CONTRACT Count      ACTIVATIONS_RATIO
22566                            16299                      1.38450211669428

Who can tell me the root cause? thanks.