Norton Online Backup saying no more space when I still have free space

hi,

 

My Norton online backup says I do not have enough space to store my files.

When I open up Norton Internet Security is says I have 7GB of storage space available and 4.73 used..  On average, each of my backups has been 2.59gb

I ran a capacity check and it is now saying I have 6gb of data needing backup (none of my backup file selections have changed nor have I added anything to change what is on my computer) and only 4gb available and should purchase 2.2gb more

Why is this happening?

Is it backing up each date and saving it or overwriting the previous backups.  If it is backing up each date, how do I change it to only take a snapshot of my computer at the time of the latest backup?

Is this a gimmick Norton uses to get people to buy more space?

 

Thank you!

Sarah