It was decided on the mailing list that the 'maintainer' field should be renamed to 'maintainers' to make it clear that there can be more than one. Note the old 'maintainer' field was capable of being a single entity or a list just fine.
So for backward compatability, 'maintainer' is preserved, 'maintainers' is added and the actual value used is the union of the two, with duplicates removed.
On the charm page the heading has been changed to 'Maintainer(s)" and an unordered list is used to present the names.