Several reasons, they may be part of an agri-environment scheme, where the access paths and/or areas may are not shown on the OS map, just on the Natural England website.
This is the case in Wales where permissive access done under the Tir Gofal scheme are not marked on OS maps, because they are only open for the duration of the agri-environment scheme.
Perhaps OS maps are too expensive to be re-surveyed every few years? How long is it between OS maps being updated?
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