DB2 Query use nonmatching index scan

DB2 Query use nonmatching index scan

I have table which has 6 million rows and 63 parts and one partitioned clustered/unique index with 3 cols. runstats are uptodate and DB2 V10 on z/os

When I use simple select from this table like below
SELECT X.*
FROM T1 X
WHERE X.C1 = ?
AND X.C2 = ?
AND X.C3 BETWEEN 1 AND 2000
WITH UR
It showed matching 3 cols index access
But when I use
SELECT X.*
FROM T1 X
WHERE X.C1 = '999123'
AND X.C2 = '657890'
AND X.C3 BETWEEN 1 AND 2000
WITH UR

It showed nonmatching index scan..
Can any one tell use why this not using Matching index .

Thank you all

Sri