Mini ATX is a proprietary form factor. There is no widely accepted mini ATX form factor used by motherboard or case manufacturers. You might be thinking of mini ITX, which is smaller than micro ATX.
Mini ITX is a miniature ITX form factor. It is smaller than ITX. It has nothing to do with ATX, whether mini or micro.
There is EATX, ATX, micro ATX, ITX, mini ITX, nano ITX, pico ITX and mobile ITX as well as a few oddball proprietary form factors like mini ATX that are used only on proprietary systems. Mini ATX is primarily only used by AOpen Inc. HP, Dell and others have their own proprietary form factors as well.
Happens easily, and most don't realize that. There are quite a few form factors that the average person will never see or have any reason to know about. ATX and microATX are 90% of the aftermarket, although miniITX and other Small Form Factors have a good niche foothold. Then you have NUC, Intel's Next unit of Computing form factor that is basically a small raspberri pi sized case with full on working Intel hardware inside. Expensive. Would be great for a lot of workstations if the prices weren't ridiculous.