Should agency get a cut of overtime bonus?

I take people’s point about the agency’s arrangement being none of my business. However, on the other hand, I’ve been at current client for eight months and it chafes a little knowing the guy in the truck next to me is getting 50% more than me after eight hours. At this stage, the agency do nothing, I deal completely with the end client (although admittedly they sourced the job for me).