In my experince, site injections work(amazingly), but only with non-esterified AAS. What variant of Test are you using? Unless its Test suspension, I would bother with sites... just stick to glutes.
To answer your question: When you use esterified Test(propionate, cypionate, enanthate, etc)... before your body can use it, it has to cleave the ester off of the Testosterone. This requires the action of an enzyme called 'esterase'. The only place esterase is found is in the blood. So... the Test your injecting IM, has to be absorbed into the blood stream before your body can use it. I don't believe a big wad of oil sitting in your bi, tri, or delt is going to make you stronger.
Proponents of Synthol will disagree with what I've told you. I respect the fact that others will have different opinions than I do... I'm just telling you what I know form my own experience.