This is the difference between the expected time of arrival of some data, and the real time of arrival of that data. You be tempted to think of this as tolerance, but it isn’t. Tolerance means to compensate for jitter, or “de-jitter”.

Most FDC's incorporate some kind of jitter hardware so that they are tolerant within some boundaries if the Bit Cell widths are not quite the ones expected within a given amount of time, i.e. the clock speed selected for cell width. This is why it is possible for the FDC to read bit cells that are not exactly the same width as expected - and indeed they are not usually the width expected, due to the nature of magnetic recording.

This fact was well used by Copy Protection designers who made the bit cells different widths on purpose, something that could only be reproduced by special mastering hardware, and due to the tolerance built into the FDC‘s, the data could be read properly, it was just impossible to write by the user. This hardware is built into most FDC‘s, and what you normally read using the FDC is not actually the same as the data as it is present on the disk surface.

This is a very important fact to bear in mind when preserving magnetic media! For preservation, you need to insure that you get 1:1 what is on the disk surface, NOT what the FDC interprets it as. For any system.