Phil, the appearance of the paint in the bottle is a poor guide to the color as it will be when thoroughly mixed and applied to the surface. However:
The pigments used by paint manufacturers change, sometimes with every shipment. This has to do with what supplier has the least expensive material.
Binders also change, sometimes necessitating a change in pigments.
When properly done, these changes lead to little change in the color as applied, since test batches, and production batches, are tested and shaded to a standard. While shading, even (or especially) computer aided, is non-trivial, the results are variations that should not be particularly obvious. There are two exceptions: sometimes the changes mentioned above result in a color that cannot be made to a close match with the standard. The other is that if you compare paints manufactured at sufficiently different times (years) there may be a color drift, again due to cumulative changes in pigment and binder.
Poor manufacturing practice (but widely done) is to match one batch to one or more of the previous batches, rather than a fixed standard. The advantages to this are obvious, but it leads to significant color drift over time. It's fine if the standard is used to check the drift periodically, but sometimes that isn't done.