Strange REVOKE issue (15.7 SP110)

HI All,

I am wandering whether REVOKE has changed between 15.5 and 15.7.

I encountered a strange issue :

1> Grant Insert on dbo.amd_abo_clt to public
2> go

4> Revoke Insert on dbo.amd_abo_clt to IT_READONLY
5> go

Some or all of the specified privileges were not revoked because, they were not granted, or they were granted by some other user.

I got a message , as not previous grant has been done

1> sp_helprotect amd_abo_clt
2> go
grantor grantee type  action     object      column predicate grantable
------- ------- ----- ---------- ----------- ------ --------- ---------
dbo     public  Grant Delete     amd_abo_clt All    NULL      FALSE
dbo     public  Grant Insert     amd_abo_clt All    NULL      FALSE
dbo     public  Grant References amd_abo_clt All    NULL      FALSE
dbo     public  Grant Select     amd_abo_clt All    NULL      FALSE
dbo     public  Grant Update     amd_abo_clt All    NULL      FALSE

(1 row affected)
(return status = 0)

1> revoke all on dbo.amd_abo_clt to IT_READONLY
2> go
Some or all of the specified privileges were not revoked because, they were not granted, or they were granted by some other user.

1> sp_helprotect IT_READONLY
2> go
grantor grantee     type  action object                     column predicate grantable
------- ----------- ----- ------ -------------------------- ------ --------- ---------
dbo     IT_READONLY Grant Select amd_abonnement_archive     All    NULL      FALSE
dbo     IT_READONLY Grant Select amd_batch_ecv_mabs_def     All    NULL      FALSE

----------------------------------------------------

I need to revoke inherited permissions from public , from a database group , but  it does not work.

In 15.5 I had no messages at all.

Any idea ?

Regards

Marc