It often is arbitrary. Also, the rules as enforced by the awardee institution often differ from the funding agency's written rules. The people who are respons

It often is arbitrary. Also, the rules as enforced by the awardee institution often differ from the funding agency's written rules. The people who are responsible for reviewing purchasing are only responsible for making sure no rules are broken, not with producing anything, so they tend to interpret the rules conservatively and just say "no" if they're not sure. The IT department also may try to block anything they didn't pick even if the funding agency would allow it. (Speaking from experience with university bureaucracy.) It's not out of concern for cost-benefit ratio (overhead from parts selection and assembly) or anything like that. Getting something useful done is solely the researcher's problem.