I have not been able to determine the reason why, but sometimes you will find that your sandbox quota on your site collection does not match what Central Administration says it is.
- Instruction for viewing your sandbox quota on your site collection (Users)
- Site Actions - Site Settings
- Solutions under the Galleries header
- At the top you will see "Your resource quota is X server resources. Solutions can consume resources and may be temporarily disabled if your resource usage exceeds your quota." where X is the actual number.
- Instruction for viewing your sandbox quota on Central Administration (Admins only)
- Application Management - Configure quotas and locks
- Use the dropdown to select the site collection in question
- Under the Site Quota Information section, you will see 'Sandboxed Solutions Resource Quota'
The issue is that these numbers do not match sometimes. The number found on the site collection is the number SharePoint will actually use. To fix this problem, you need to go into Central Admin and temporarily change the quota on the site(s) in question. Change it back the value that you want and this will trigger the numbers to match correctly.