Do collections normally get compiled into "compilations?"

1
There are a couple collection's I'm keeping an eye on that have many chapters and wanted to know if it's normal that they get re-released in a complete or "3 in 1/ x in 1" format to get multiple chapters in 1 pack?

Example, Hypnotic Sexual Counseling. Basically a whole volume. Recently released compilation 1 with majority of the chapters in 1 collection. Last few you have to buy individually. Is this a normal practice? Or who decides if this happens? What factors go into releasing these compilations?
3
Yeah. Magazine chapters usually get released as a tankobon. That global for manga and comics.