It may be down to transmission times, some countries operating UK registered aircraft may not recieve Emergency AD's at the same time, so still could be operating them after the date issued if it was earlier, that would mean unbeknown they would be operating without a valid ARC and hence and Airworthiness Certificate, that then has knock on effect on Aircraft insurance and pilots flying unairworthy aircraft, so a little leeway is given so hopefully everyone is in the know to prevent the above happening.
That's my thoughts on the possible scenario.
It was issued on the 26th but my email system that gets all AD's from my service provider arrived on the 1st.