Disk space allocated to your account is 49 GB. Around 8.1 GB is used by files/folders hosted on your account and free disk is around 42GB. Hence, mysql Disk usage shows stats as 672MB/42GB (42GB is free out of 49GB since 8.1 used by account data.)
Here, mysql quota means free disk space of the account. If disk space usage goes from 8.1 to 48 GB , mysql disk space stats will be 672MB/1GB
Thank you for sharing the screenshot. @sysni**** is correct. For instance, if your account's quota is 1000M and the account currently uses 500M, then the available MySQL usage would appear as a percentage of the "available" disk space (X/500M) instead of the global disk space quota (X/1000M) assigned to the account.