The calibration will tell you the actual error at the time the calibration is carried out, you can then account for the errors in any measurements you make.
All our scopes are, of course, calibrated during final test and are guaranteed to meet the published specification for but no calibration certificate is made or issued. If a calibration certificate is requested from new, or a scope is later returned for calibration, the actual errors & accuracy for that particular scope are measured and recorded on the certificate. For example, a scope might have a published accuracy of +/-1% for voltage. A scope direct from the factory will meet or better that accuracy. A calibrated scope will have the actual error recorded for each voltage range and that error might be a reading that is 0.5% high. This error can now be allowed for when taking accurate measurements.