@Pav, We store custom fields with each link click. ie when an email is sent, custom merge fields pass additional data to the link to store with the link click. The IS link is part of the setup of that tracking link and it is forwarded to as soon as the link registers on our end. Not a single test that has been done has failed to forward successfully so I doubt it's not "getting there". I might have to export the IS report and run a NOT IN (SELECT ... on the two tables to find out what's happening but it shouldn't be having so many more link registrations especially taking into account that the DB Clicked value also includes a few tests and possibly cases where someone clicked the link in the email and only then after unsubscribed.
We did this both to independently create counts separate from IS as I've heard too many times about their numbers not being right with link clicks but also so that we could have a product agnostic way to track individual link data like email, item purchased, record id etc.
So the DB numbers, unlike what @Kathy_Lewis has indicated, are independent of IS' reporting.