I have 1.24 TB disk but it shows 500 GB

Also type diskutil cs list in Terminal and post the results.

It sort of looks like the Fusion Drive was not created with 100% of the space. And maybe the remainder that's missing is free space. Just a guess.

It looks like the Fusion Drive was not created with 100% of the space or maybe the 1TB drive isn't formatted correctly. I can't explain why your 1TB drive is only showing 268GB from the diskutil list command.

Did you have a Bootcamp partition for Windows at any time?

Below is my Fusion Drive diskutil list. My 1TB drive shows 999.3GB (disk1s2).

$ diskutil list

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *121.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 121.0 GB disk0s2

3: Apple_Boot Boot OS X 134.2 MB disk0s3

/dev/disk1 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_CoreStorage Macintosh HD 999.3 GB disk1s2

3: Apple_Boot Recovery HD 650.0 MB disk1s3

/dev/disk2 (internal, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +1.1 TB disk2

Logical Volume on disk0s2, disk1s2

C5D87019-7A9D-44EA-9C63-D8873D11B135

Unencrypted Fusion Drive

what if you open disk utility ?

FWIW, there is something definitely wrong.

Type this in the Terminal then post the result.

diskutil list