What are these markings on a Shimano FC-M770 crank?
The only people that can answer this with 100% genuine authority would be Shimano's R&amp;D team.
Clearly those marking are not there by chance so lets presume what is a reasonable reason :

Weight Saving (it's going to be a fraction of a gram at best) but some people are picky
Reduced Drag (it could be a fractional difference while spinning at 90rpm as opposed to 70rpm)
Intentionally built in to make them harder to clone in the manufacturing process

Your best bet is to get in touch with Shimano and ask them, they will either tell you or they wont.