SQL Query Analyzer don't pick correct Index if i use Variable

Hi,
Today i came across a strange issue (although i have solved it but that is not the answer of my question).
I have a table with following structure (following structure is only for question purpose, real table have about 15 columns)

CREATE TABLE TBL1
(
ID INT IDENTITY(1,1),
CODE VARCHAR(13),
CODEValue VARCHAR(50),
NROW INT)
I have 2 Indexes
1- ID is primary Key
2- NRow has a NON-Clustered Index named as IDX.

I have about 10 million records in TBL.
Now if i perform a simple search

SELECT
TOP 100
ID,CODE,CODEValue,NROW
FROM
TBL1
WHERE
NROW>567000

Result is instant and Query analyzer show it is using index IDX with Index Seek.

But if i perform following query with following code

DECLARE @NROW INT
SET @NROW=567000
SELECT
TOP 100
ID,CODE,CODEValue,NROW
FROM
TBL1
WHERE
NROW>@NROW
System takes 6~7 sec and use Primary Key as Index and perform TABLE scan.
But if i change my query and use INDEX HINT as follows
DECLARE @NROW INT
SET @NROW=567000
SELECT
TOP 100
ID,CODE,CODEValue,NROW
FROM
TBL1 WITH (INDEX(IDX))
WHERE
NROW>@NROW
Result is instant and Query Analyzer showing Index Seek using IDX index.

I am not sure why SQL is not picking correct Index, any input?

Read the articles about parameter sniffing to get the answer...
http://www.google.com/search?hl=en&...&q=sql server 2005 parameter sniffing&spell=1