In some cases these are used to get people to join the platform as much as share actual content.
Have a search through your SPAM and email as a whole (inbox/deleted etc) to see if anything with Instagram is in there. It might be you have had an invite in the past which you have ignored which is where your ID is locked against?
What might have happened is that some Insta program/process has harvested your email address from another persons contacts. If they have then sent a "i am on insta, you should join me" type request (or more likely insta has using their ID) it has created and held your email address in a pending state until you join. This would result in your name being in a pending state, it has been reserved by the process but not completed.
Ask any of your friends on Insta to do a search against your email ID/name from within Insta to see if you show on anything, also search for your name in general on there.
Whilst I agree with your aversion to having your details spread across many sites you should also consider it this way. If you do not have your ID now then it can be cloned by someone else. I do not use Insta however I did create an account on it when it started up using my "non-valid credentials" so others could not steal my ID. I have a set of wrong credentials (DoB, Address etc) linked to a gmail account I monitor which I use for all stuff that is likely to get abused. So if I want to protect myself I get in there first using these incorrect credentials. I also save them to a password protected note on my iPhone when I create them just for security. Doing this allows me to avoid risk as far as possible and also observe where my name seems to be getting leaked from possibly. I prefer to know rather than worry and so having the account up front avoids anyone else being able to take advantage.
I do keep a list of all of these "phantom" accounts. Please excuse me as this is a bit horrible but in truth saves a massive amount of pain. In the event that you were to pass away the workload involved in tidying up someone's on line footprint can be quit debilitating for the people left behind whilst they are trying to deal with the loss. Completing probate and then finding outstanding payments and all sorts of other stuff can bring more suffering as it drags it all up again. The Interweb leads to a fire and forget approach but in truth it can be very very messy for a number of reasons.
