I think the pacing is weird because there's a lot of stuff happening everywhere when for most of the manga, it was focused pretty much only on Itami and his immediate circle with the odd chapter or end of volume bonus to develop the rest of the cast and setting. Because of how much is happening...
The enumeration is off compared to the official releases (When Johnny Comes Marching Home is 79 in the tanks), but there are ten chapters that haven't been translated.
@puta Its to deal with the shockwave-- opening your mouth keeps the air from forcing its way into your lungs, which'd collapse them, and covering your ears keeps your eardrums from getting ruptured.