Why is there a slightly enlarged shoulder at the top of the bolt that secures the cam chain tensioner?

 

Gots ta be precise...
​​​​​The size and shape of that bolt, at the end O the day, will makes sure that the tensioner ends up bolted down in the exact right place... Hard to explain, but the shaft itself would allow for a lot of wiggle room, that slightly wider section at the top of the head of the bolt guides it into the right position on torque down

 

I can see how that would work - does it need a corresponding space to fit into - which I didn't notice on the parts I fitted or does something deform to accommodate the extra metal?

The ones I took out don't seem to have the same shoulder but the whole design is somewhat different.