Areva/GE/Master-Lee/Whoever is a different world altogether from Bartlett.
If you work a hard-money job, the customer doesn't give a rodent's hindquarters how much you get paid - or if you are even there to begin with. The local pay rate is totally immaterial. Your employer determines what to pay you based on what they can afford.
When you work for Bartlett, you are doing "staff augmentation". Basically, they don't pay for a specific output, but for your services at an hourly rate. In this case, your rate of pay is the major factor in the cost of the contract. Therefore, the customer actually cares how much you are paid. They put it into the contract, which they negotiate individually. Since every contract is different, every pay rate is different.
Bartlett doesn't have enough profit built into their contracts to be able to standardize pay rates - unless they made them all really low. So, be careful what you ask for, you might get it.