Why such a tiny difference in crankarm length?
MTB crank arms usualy come in 165, 170, 175 mm. Some people (usually PROs) can have access to more fine grained lengths (e.g 167.5) or ever smaller sizes such as 160mm.

The reasons for such "micro-optimization" are:

clearance

Today MTBs for trail riding need to have a lot of front and rear suspension and very low bottom brackets. This makes it easier to hit rocks with the pedal and that is not desired.

Also note that ground clearence problems are in addition being solved with very thin pedals (thin pedals also solve other issues).

So, saving 5mm from the cranks and 5mm from the pedals may not seem enough but it can actually reduce the times you hit the ground.

Note that downhill specific bikes will usually have 165mm cranks.

proper fit to rider height

A short rider (with short legs) will pedal more efficiently on a 170mm crank than a 175mm one.

Thus small MTBs usually ship with 170mm and medium ones with 175mm.