Converting Datetimeoffset Field To Datetime Field / Why Milliseconds Value Is Incorrect

DECLARE @datetimeoffset datetimeoffset(3)
DECLARE @datetime datetime
SELECT @datetimeoffset = '2012-11-08T17:22:13.575+00:00'
SELECT @datetime = @datetimeoffset
SELECT @datetimeoffset AS '@datetimeoffset ', @datetime AS 'datetime'
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Result of above SQL is
@datetimeoffset datetime
2012-11-08 17:22:13.575 +00:002012-11-08 17:22:13.577
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The result should be '2012-11-08 17:22:13.575', why the milliseconds value is incorrect