They are the same in 2010 because that's where the graph starts. The rest of the graph should be interpreted as "cumulative growth as of $DATE, relative to 2010".
This is actually a common way of presenting two stocks. I believe Google Finance does it. Search for any ticker symbol (e.g. GOOG) and then type in another where it says "compare" (e.g. TWTR). No matter how far back you pull the graph, the leftmost points on each line will always overlap.
I agree that it's a bit weird if you're not used to it.