Jac, FWIW you may find this excerpt from an email response I got from Ilford some time back to be of interest:
...there can be small differences between 35mm, 120 and sheet film as these do use different bases and slightly different emulsions. Although we endeavor to manufacture all three types to be as close as possible in order for development times to be consistent across the range for any one film type (and they are for most practical purposes) someone carrying out the exacting type of testing you describe will inevitably pick up small differences.
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