Congratulation for your Nominations.
The error can be due to two main reasons. Firstly, that it is simply a technical error as many faces it regularly and secondly, due to maximum points that can be awarded.
You may have claimed 15 points for sibling and then received Nomination giving you 600 points. However, maximum additional points that can be added are 600, not 615. Additional points include siblings points, French Language, Degree in Canada, Arranged Employment, and PN points.
If you think that the above doesn't apply to you and you should have received 600 points for Nominations then you can report the technical problem using your profile.
I hope this will help.