EDIT: Come to think of it, I do have Games & Rounds on my ignore board preferences. If this is indeed the reason, wouldn't it still make sense to have the topics appear in my history considering that I was the one that created them? One thing is to want to ignore a particular board, but considering that I was the one that created the topic, I would have interest in following that particular topic of discussion.
Yes, that's the reason! The ignore board preferences function is designed to completely filter out content from those boards, including topics you created.
While you may have a point about wanting to easily see all your created topics, there's a trade-off to consider. Since the search feature allows you to list topics from any member (by changing the uid in the URL), including topics from ignored boards would defeat the purpose of the ignore function. Besides, I don't think there are many members who create topics in ignored boards on a regular basis.