Under the Code of Recommended Practice for Local Authority Publicity, local authorities are to take particular care as to publicity in sensitive periods such as those preceding an election. In councils this period is taken to be the period after which notice of elections has been given, which must be at least 25 working days before the election date.
For central government, there is a long established convention that for a period preceding local elections particular care is taken in the conduct of government business, including but not limited to publicity. This period, customarily known as the election purdah period, is traditionally for three weeks before the local elections.
May's local elections are for local government, and are not elections for central government. It is not unreasonable therefore that more restricted rules are in place for the public body which is actually having the elections.
By contrast, for parliamentary elections, purdah restrictions commence from the day that the general election is called. In this context, the principles are not substantively different from local government.