Yes. About 20% of people with glaucoma will have normal pressures.
Yes. While "normal IOP" is 10-21, up to 20% of glaucoma is "normal tension glaucoma, " where optic nerve damage and peripheral visual field loss occur even with an IOP in this range.
Yes. Although increased eye pressure (iop) is the biggest risk factor for developing glaucoma, it's not the only factor. Individuals can develop glaucoma even with modest iop. Other important risk factors that increase risk for developing glaucoma are family history, certain ethnicity (African American, Hispanic, Asian, Japanese), thin corneas, age, Hi BP, Lo BP, xs far- or near-sightedness.
Yes. Patients with glaucoma without increased eye pressure are commonly called "normal tension glaucoma" or "low tension glaucoma." glaucoma is multifactorial and probably involves more than just eye pressure, i.e. Ocular perfusion pressure, neurosusceptibility, etc.