The odds of there being 7 helms and they are all Leoric's is, let's say, .0000312%.
But the odds that at some point in your entire D3 lifetime finding 7 Leoric's in a row is a complicated problem. http://www.drdobbs.com/architecture-and-design/20-heads-in-a-row-what-are-the-odds/229300217 discusses this.
I used a similar formula in a program I wrote a long time ago. I found that the odds of finding 7 Leoric's in a row somewhere within 100 legendary drops is 0.1%. 1/1000. So if you've been playing a while and the number of legendaries you've seen is more than 100, it's higher than 0.1%.