Reviews seem to be done on equipment that was outright purchased by the staff, or equipment that was donated, or loaned, for the review. Obviously, the staff's budgets prohibit them from buying every piece of equipment, and the manufacturers, for various (often petty political) reasons do not let everybody review their equipment.
By the way, some manufacturers have the opinion that they only need ONE good review of their product, after they have received that ONE good review they pull the unit from the review market.