As is often the case, the issue revolves around the specific wording of the regulations. Ferrari and Renault successfully argued that, contrary to its spirit, the letter of the law (or regulations, in any event) didn't actually specify when existing engine suppliers had to complete their revisions for the upcoming championship. The thing is that the rules were more clear when it came to new suppliers, so Honda was told that it would have to complete its design before the start of the season – unlike Ferrari, Renault and Mercedes, which would be allowed to continue development (albeit on a limited basis) throughout the year.
Recognizing the inherent injustice of the resulting regulations, the FIA has consented to Honda's request that it be afforded the same opportunities as its rivals. The Japanese manufacturer, which returns to the paddock this season with McLaren, will therefore be allowed to make adjustments to its engines as its first season back on the grid progresses, just like the other engine suppliers.