The Mekhilta explores a subtle legal distinction between two types of compensation: ripui (medical expenses) and sheveth (work-disability payment). When it comes to medical expenses, there is no distinction between treating the original wound and treating complications that arise from it. If a wound becomes infected or spawns secondary ailments, the responsible party pays for all of it.
One might think the same unlimited scope applies to sheveth — work-disability payments. If the wound leads to long-term consequences that prevent the victim from working months or years later, perhaps the striker should pay for all lost income indefinitely, just as he pays for all medical treatment.
The Torah blocks this extension: "But his sheveth shall he give." The word "his" — referring specifically to the sheveth — creates a boundary. There is a distinction in sheveth that does not exist in ripui. Work-disability payments are limited in a way that medical payments are not.
The practical effect is that the striker pays medical expenses for everything connected to the wound, no matter how far the chain of complications extends. But work-disability payments are more narrowly tied to the direct, immediate impact of the injury on the victim's ability to earn a living. The Mekhilta draws a line that keeps medical obligations broad while keeping work-disability obligations focused. Both types of compensation come from the same event, but they operate under different rules of scope.