You're just overthinking it, which is something people commonly do in the PED world. Stop over thinking things.
All you need to know is that IGF-1 (or ANY hormone, for that matter) will quickly bind to receptors on cells that are a PRIORITY and require it THE MOST first. This means that through training stimuli, the receptors on the skeletal muscle you recently worked will be in a highly excited state and very receptive to IGF-1, so the IGF-1 you inject (whether it is subcutaneously or intramuscularly) will go there first for the most part. If you were to chronically administer IGF-1 into a lazy couch potato who doesn't work out, almost all the IGF-1 would be binding to receptor sites in cells other than skeletal muscle, because that person' skeletal muscle is not in need of IGF-1 due to the fact that the person is not triggering priority for it through a training stimulus. That person would most likely end up with organ and gut growth almost exclusively.
Yes, SOME of it will go to the muscle it is injected in first, but it is so minimal I honestly do not feel it would make a difference. The manner by which anything injected into a muscle works is this: you inject whatever it is into the muscle. Okay, fine. But it doesn't just start attaching to receptors in muscle tissue right THERE where it landed! It needs to seep into the tiny capillaries, and then get pumped to the veins and arteries and enter circulation so that it can then find muscle cell receptors to bind to.
Here's a good analogy:
I'm going to paradrop your car (with you inside it) into a populated suburb of a city. Your goal is to drive to a house and park in its driveway (and ONLY in the driveway of houses that contain spaces to take in cars!). Now, the odds of your car landing RIGHT ON A STREET/SIDEROAD/RESIDENTIAL ROAD is slim to none (after all, roads = capillaries, arteries, and veins... and we aspirate so we DONT inject directly into them). So with that being said, your car with you in it, slowly lands onto the football field of a school. You need to then drive the car OFF the field, on to the street, and then drive through the streets to a house and park in its driveway. Now imagine 100 cars being paradropped with this goal, and only certain houses have availability in their driveways, as I said. Some local houses will receive cars first, yes, but if there are open and available driveways in houses that are needing cars in other subdivisions first, they will go there.