You just started a new training program. It is well designed and incorporates all of the movement patterns (horizontal push and vertical pull one day, vertical push and horizontal pull the other. Squat/split stance one day, hinge/single leg the other, for an example) split in to two separate workouts, with some purposeful corrective work tossed in between rest periods.
You have a training journal, which you keep updated daily. Some of you may even go so far as to record your psychological and emotional state pre- and post-training session. You're training five days a week-three strength training and two more conditioning focused days.
Your diet is such that it supports strength and lean muscle gain while simultaneously maintaining or decreasing your body weight, but not increasing it.
Everything is going great, and then four weeks later… you change it up for a new batch of exercises and a new set/rep scheme, because that's what you're supposed to do, right? Four weeks later, you switch it up again, and then again, and again. "I know!", you think to yourself. "I'll try out the same program I started a few months ago. I bet I've gained a ton of strength since then, since I've been strength training all this time!! Yeah!". To your dismay and confusion, you aren't any stronger in your lifts.
Unfortunately, you have just become a perfect example of someone who doesn't have a clue what the SAID principle is, or why it's important. In the most simple of terms, the SAID principle (Specific Adaptations to Imposed Demands) means that if you do the same crap all of the time, you're going to get really good at doing that crap.
Bill Gates unknowingly used the SAID principle when he began computer programming back in school. He wanted to get better at programming. What did he do? He PROGRAMMED! He didn't play sports one day, watch a few movies the next, or play board games the following day. He programmed every day he possibly could, which was quite often, actually. By the time he was of college age, he had amassed over 10,000 hours of time honing that skill.
What does Bill Gates have to do with the SAID principle as it relates to fitness, then? Well, he practiced something over and over again until he had mastered it. Likewise, in strength training, if you treat it just as an activity - something recreational to do like a pick up game of football or playing a board game - it will never amount to much in terms of results. But if you have a program of a specific few exercises that you do on a regular basis, well, chances are you are going to make a significant chunk of progress in improving your strength in those movements over time. For example, if Darwin wanted to improve his deadlifts, he had damn well better be deadlifting! Remember, Darwin, you are improving movements, not just strengthening muscles! Darwin doesn't need back hyper-extensions or hamstring curls. She just needs to deadlift, an exercise that will improve her hip hinging ability while simultaneously strengthening all of the muscles the above exercises would have, while also strengthening her joints and connective tissue (though the movements above can be useful as accesories!). Only then will Jennifer see the kind of progress she wants to see, and it comes from consistency, and frequency of practice.
Moral of the story: stick to your programming and focus on the main lifts. In turn, your strength and efficiency will go through the roof!