User mismatches

I have set this up on our main site and new website.

We have several hundred users on the main site however this plugin seems to rely on them logging into the new site and creates userid mismatches if they do not and then someone else either logs in or registers.

Surely this should be creating the users using the same details?

For example.

User signs up at the main site and has the ID of 3000 but does not use the single sign on yet.

Second user signs up at the main site and has the ID of 3001 then uses the single signon to the second site. This then creates the user on the second site as 3000 causing a mismatch in the details between the two sites.