Difference between two cleavers - straight back vs curved
So, as I couldn't find anything online regarding this. I asked the butcher I work with.

He also can't give a definite answer, he does however have a collection, including a cleaver similar to the Curved Front. He recons as the front rarely takes a pounding like the main flat section, it's almost always sharper. As a result if for example he is cleaning the meat off Beef Ribs for mincing. He'd use the front part for slicing down the bone then the main section to chop any bone for other uses (Gravy etc). He does point out it's rare he would do this as he almost always has a boning knife to hand so just uses that, but if in a rush to do something for a customer it's just handy. 

Again this is what he uses the front for, not necessarily the intended purpose. A reviewer for that particular knife seems to use the front for cutting pizza... hmm. (here)