Drop in SERPs as I widen the topics

A couple of things could cause a small drop like that...

Are you using a dynamic home page where all of your posts show up there and roll off as new posts are added? If you are, you probably pushed your phone related posts down the page, making their links weaker. A static page is preferable in most cases.

If you added new categories, they are likely linked to from your navigation menu. You diluted the strength of every internal link on your website when you did that. Nothing to worry about, but that would cause a drop of a few spots in a lot of niches.

Mike has pretty much covered it above, when new content is added to a site if it's 'shown' on old parts of the site it will impact their SERPs.

For example if you home page was tightly niched to phone SERPs would mean everything about the page screams rank me for phone SERPs. By adding tablet articles if that content is loaded on the home page (if WordPress you'll see the posts archived there) it's saying rank me for phone SERPs and Tablet SERPs.

Although you can in theory achieve both, it's harder to target multiple niches on a single webpage than target one.

Add to this new content means your link benefit has to be spread further, if your content is automatically linked sitewide from Recent Post widgets or navigation menus your currently ranked phone articles are now linking out to webpages related to Tablets. Your phone webpages used to only link to other pages about phones, now they link to pages about phones and tablets.

It's one of those balancing acts adding new content in different niches, it's going to have a negative impact on current niches, but can bring in new traffic. You can try to minimize the damage by creating silo link structures so only related webpages link together, but even that won't completely remove all damage to current SERPs unless when adding new content you also add new backlinks so the old content still has the same amount of PageRank overall.

David

Silo your website, and keep making and promoting your content. As the website increases in authority you'll probably regain those rankings.

Haven't you thought about the possibility of your website dropping even without adding articles that deviate from the main topic of your site?

It is because the internal link juice was distributed towards more posts (the new posts you created)....