See FIG 1
for a good pic on how an old fashioned injector pump / injector works.
Fuel from the pump is delivered to the injector at 1500psi + .
The injector piston [or spool if you prefer the part beneath the spring anyway] is a good fit in its cylinder but still leaks a small part of the fuel into the the very top area connected to the return .
This leakage MUST occur to keep things lubricated. How much they leak is not that important as long as they all leak approx. the same amount. Hence the manufacturers rebuildes supplying matched sets of injectors.