It is not a good practice to have deviations in Batch manufcaturing. I agree with Brian regarding the allowed +/- 10% deviations at times in Industry.
We have to understand when such deviation occurs the unit operations like Mixing/Blending/Drying and other parameters get effected. The reason is due to the fact that the R&D and Production start-up team will optimize all these parameters and keep a proper record of Critical control points. At these points the parameters will varie when there is swing either way in quantities. In such casses of deviations it is always risky to release the batch immediately. This batch should go an extended testing for all parameters.
The critical control points must be selected and extended and comprehensive testing must be done at each stage before it gets a release slip to pass to next stage. This batch becomes very crtical for the company.
Most of deviations happen when active ingredient is less than the approved batch size. This happens when an avialbale API or Excipient is found short in stores. The production head takes batch with available material.
The reason for a batch deviation also happens as per requirement of market demad. It might be a 5% or 10% more than actual batch size.
These deviations at times ruin the reputation of company as shop floor production chemists will have a little or no knowledge about how Btach sizes are validated and fixed. It is the responsibility of QA and Compliance department to hold the batch for extended testing and release such batches.
Manufacturing chemists should not have a preconcieved notions about Batch size deviations. If they take it granted it will ruin total compliance procedures.
Technically spaeking no where the Batch should deviate. The reason is Btach sizes are fixed by industries depending up on multiple factors like Capacities, Products, Parameters, Market demands and stability data vaialble from R&D and Pilot studies.