The difference is that our normal MT uses Syncros to match the speed of gears during shifting for dogbox there's no syncro and is the dog teeth that simply slot into the drive gear like a coupling.

There are companies out there making helical or straight cut sets for dogbox also. So the misconception is that all dogbox uses straight cuts gear.

Why straight cut gears? because helical gears puts thrust loading on the bearing and shaft due to helical gears teeth are machined with an angle so when they turn, they will try to moved away from each other. But straight cuts gears will create more noise and gearshift is quite rough. Other from that, because there's no syncro on dog box the gears and gear teeth can be made thicker and hence stronger. 

Also dogbox gears set are fully submerged in oil compare to normal MT that uses the rotating action of the gears to lubricate the components.

See the picture below and notice the big dog teeth like bump on the side of the gears? those are dog teeth.


PS. MT users if you notice why during reverse you heard that loud whinning sound....because the reverse gear is straight cut. So basically that's how straight cut gear sounds like. The faster you reverse the louder the whine.