Problem with user meta

Hello,
I've created a form frontend for Student registration, when my form is submitted, I save all user meta.
My problem is with wpcf checkbox,
I set my meta_key to 'wpcf-interests' and the value to a:2:{s:64:"wpcf-fields-checkboxes-option-49595e1e4d3ffb579e7fa2c39ca8fe5b-1";s:10:"automotive";s:64:"wpcf-fields-checkboxes-option-afbfcb3c80f9dbc1a4970d2fc94009a6-1";s:6:"travel";}
and call add_user_meta.
When I look into the database my value is setting as : s:181:"a:2:{s:64:"wpcf-fields-checkboxes-option-49595e1e4d3ffb579e7fa2c39ca8fe5b-1";s:10:"automotive";s:64:"wpcf-fields-checkboxes-option-afbfcb3c80f9dbc1a4970d2fc94009a6-1";s:6:"travel";}";

I've try to removing action wpcf_admin_user_profile_save_hook but without success.

Which action is called when we use the add_user_meta ?

Thanks for your help.

Best regards
Sébastien