It's complicated and there are a lot of reasons it happens.
For one not all surgeons are created equal. Some are a lot better than others and the work they do shows it.
A lot of the people that get implants want to do it cheep, and it's one of those things that you get what you pay for most of the time.
Some women's body types is just not built to take the large implants, but they get them anyhow and it just doesn’t work out. There is only so far you can push it and have the possibility of looking more natural.
Some women just don't do the research to find out what looks good or even how it will look on them when it's all over. The either don't know what they want or don't realize how it will look after it's all done. They just want it done.
Often the size of the breast before implants has a determination of how natural they will look. Often the larger they were naturally the better they will look after implants are put in. The doctors have more to work with in that case also.
Like you said there is more than one method of putting in implants like over and under the muscle and techniques where they don't fill the implant all the way up among others. Not only do some techniques word better in some situations, but doctors have varying levels of knowledge and skill at implementing them.
Some doctors are better at leaving smaller and less noticeable scars than others.
Often the strength of an individuals body parts which can very can determent how the implants last through the years. Also how women take care of the implants can determine how well they last. Ideally they shouldn't be bouncing around all over the place, and in something like porn that's not always easy to do.
I also wouldn't be surprised if things like implant manufacturer or the general health of the women that gets them has an effect on it either.