How can a crystal habit be round?
Botryoidal habit describes the arrangement of a large group of hematite crystals.  

While hematite's trigonal crystal system describes the geometry of a single unit crystal of hematite,  the two terms are really not related at all. Many minerals with different crystal systems can have a botryoidal habit. 

The difference between a Botryoidal habit and mineral's unit crystal geometry is a difference in scale. A botryoidal habit can be measured in inches to feet while crystal geometry is measured in Ångstroms.