The short answer is that createmultisig basically just combines the inputs and then outputs the constructed MultiSig (ie. redeemscript and address etc) onto the console...
Whereas, addmultisigaddress takes the inputs, constructs the MultiSig... and then stores it all in your wallet file for you.
This is why "ismine" is "True" after using addmultsigaddress, because it is now actually stored in your wallet. Whereas with createmultisig, it has just output the information for you, but nothing is saved to your wallet file, so "ismine" = false.
Thak you for your well understood answer.
I understood the difference between createmultisig command and addmultisigaddress command.
So I hava one more question.
If I create multisigaddress by using createmultisig command, is it true that nobodoy can get both privatekeys and redeemscript associated with this multisigaddress from this coins server ?If so, this command is much more secure so log as I remember these privatekeys and redeemscript.