No, that's the normal behaviour. Spam and offensive flags are functionally the same, the only difference being how the labelled data is used:
How does the Spam flag differ from the Offensive flag?
In terms of getting the post deleted, there is no functional difference aside from separate counts - 3/6 of either will be sufficient to delete. Spam flags provide data for spam-filtering, and offensive flags similarly provide data for anti-trolling measures.
The notice placed over a deleted post also says much the same (spam or offensive):
