I don't really know, I'd have to guess. My guess is that, with a single dose, there's a substantial period of time that there's an acutal depot of the oil in the muscle which releases the compound by simple diffusion. This partly depends on the surface area of the depot, among other things. The oil will spread as a very thin layer across the length of the muscle fibers, yielding a large surface area.
After a couple of days, this depot will become smaller and smaller, and finally no trace of an actual oil depot in the muscle can be found: the oil will have spread throughout the body and (I suppose) be incorporated in adipose tissue. The steroid will then be dissolved in a larger volume of oil (that of the adipocytes), but with a relatively smaller surface area and thus a slower rate of diffusion. (In absolute terms, of course, the surface area will be larger-but the steroid will be a lot less concentrated.)
This will be increasingly more relevant with "slow" esters, as with shorter/faster esters most of the steroid will already have left the oil depot before it's largely added to adipose tissue. And thus also more relevant with multiple doses, as there's more accumulation.
Edit: this is mostly a brain fart from my side, as I haven't thought this through too much. So perhaps I'll be slaughtered by some nerd with my logic here soon enough.