Twenty years ago, per diem was built into the hourly billing rate for the first 40 hours. A contract might have been paying $13 and hour and $50/day. The billing rate would be something like $26 (including $8.75 for per diem) and the overtime billing rate would actually be LOWER than the straight time rate because per diem and all administrative costs, including benefits, were built into the first 40 hours. That all changed a LOOOONG time ago. Since the early 90's just about every contract was bid with per diem and travel as direct pass through without markup.
Sure, under the old system, you could work 40 hours in a week (say 12 hours a day M-W and 4 hours to WBC on Thursday) and only get maybe 3 or 4 days of diem - leaving $150 to $200 for the company to keep. On the other side of that coin, you could be working 8 hour days and get laid-off Monday morning. That would leave the company on the hook for a whole day's per diem without collecting enough straight time that day to cover it. They still have pretty much the same admin and benefits costs whether you worked an hour that week or 84.
Basically, it comes down to this - you get paid what you agreed to work for. The running of the company's finances is really nobody's business but their own. If you could have all the money that Bartlett billed for your time, and you would also have to spend all the money that Bartlett pays out for things like taxes, workers' comp, state and federal unemployment insurance, benefits, admin costs, interest on their lines of credit, PPE, legal fees, payroll taxes, ... etc. you would not be any better off than you are now.
A Site Supervisor (or Site Coordinator) knows exactly as much about the running of the office as any Junior Tech on his or her site. So, don't give too much weight to some sea story just because the clerk (which is what a SC really is) is telling it.