Briefly, think of it in the lines of isolation.
In general, say a person goes to the gym and can perform a bench press without good form at 250 pounds. Now they go to an incline fly machine which they are strapped in and not allowed to use their arms in any way, only their upper pecs are activated in the movement. They are now only able to perform with 30 pounds; this is mainly due to isolation. Maybe with incorrect form in the fly machine, they can use 40 pounds, but even there, it is much less than using a grouping of muscles.