Why does some glass break sharply and other glass doesn't?

A few weeks ago, I stepped on a picture frame, and the glass shattered. There were clothes over it, so I didn't cut my foot, but when I went to pick up the shards, I cut my finger. Just today, I stepped directly on another frame. The glass shattered, and I could feel it poking the bottom of my foot, but it didn't even scratch me. I was just wondering what the difference might be. Like, if there are different types of glass, or anything else. Thanks