Why don't BMX plugs fit on MTB riser bar? In a previous question, I have been told that the grip diameter of a MTB riser bar is 22.2mm. From research I did, the grip diameter of BMX handlebars is also 22.2mm. In fact, it's possible to use BMX grips on MTB handlebars. I thought this would allow to use also BMX bar plugs on MTB riser bars. As I experienced first hand, this is not the case, see picture below.

Is that because the internal diameter of MTB bars is smaller than in BMX bars?
The outer diameter of bars is the same. Internal diameter can vary a bit between manufacturers, but you're going to have an ID of around 20 mm for road bars and 17 mm for mountain bars (source). A BMX bar is somewhere in between -- if you get the right MTB bar and the right BMX plugs, they'll probably work. 

That being said, if you can't get those in with a bit of twisting and turning, you could probably shave them off. Or, go for an expander style plug.