As with any injection it takes more time for the muscle tissue (which it's injected to) to absorb the oily solution then it takes for the gut to digest the oral form. Note that the injection site also matters to some degree to determine the exact half life (Delts will take less to absorb then glute for instance), I would estimate a 50-100% higher half life in the injection form then in the oral form

How ever the peak concentration after injection is reached quite fast and within hours, it's simply the extra time which take to the tissue to "absorb" all of the oil which prolonged to some degree the general half life