Single Opt In - The user registers his profile and email address and hits submit. - technically you will be paid on this, unless the program sees that the profiles and email addresses are all fake or created by you. - we, the programs only want to pay for real users that are going to become paying members at normal rates.
Double Opt In - After the users creates and submits his profile and email address, he is asked to check his email box to click on a link to verify their email address. This allows the program to properly email market the member, to assure that we can convert as many of them as we can, to paying members.
Remember, all programs need to convert members to paying members, or else we don't make money.
We are all in business to make money.
SOI is usually around 1/2 of what DOI is... Give or take a bit.