If I remember correctly, ‘Long’ time back when OneDrive was called SkyDrive and Microsoft account was called Windows Live account, at that time new users used to get 25 GB of free space. Later it was changed to 7 GB and then it was 15 GB. So those users who had 25 GB, their free space was technically reduced to 15 GB to make it uniform and 10 GB was shown as loyalty bonus. So those users didn’t loose anything and continued with 25 GB in total.