it depends on what tags I use, iPad is a popular one obviously.
Also, soemtimes I'll see a like of an ancient post with no rhyme to why.
I'll agree though that a good percentage of likes / reblogs are spam / bots. They also often have ads on the blog so whoever is running it will get credit when you visit the blog to see who just liked / followed / reblogged you.
One friend of mine runs a Tumblr on design, gets over 15,000 pageviews a month. He says that he gets 3 to 10 followers/likes who are spam a day. He blocks all of them to avoid linking his site to websites with plenty of ads or porn. It seems something is going on, it needs to be sorted out immediately.