The problem is that strength gain rarely, if ever, follow a linear progression. Well, it can when someone is a beginner or for brief periods of time. But if you look at the big picture, progression over 6-12 months (or more), progression will normally come in spurts. You will have periods where strength increases fairly rapidly then it will stagnate for a few weeks before (hopefully) resuming again.
I always give the 5lbs example. Let’s say that you add 5lbs per week on your big lifts. Sounds easy, it’s not much. But that is 260lbs per year. So if you start with a 225lbs bench press you will be lifting 485lbs after one year, 745 after 2 years, etc.
Heck, even if you only add 2.5lbs per week… a measily 1.25lbs per side, that would still take you from 225 to 355 in one year and to 485lbs in two years. Just for fun ask someone who benches a legit 485lbs and ask him if it took him only 18-24 months to accomplish that.
My lifetime best bench press was 445lbs. Going from my frist 315lbs bench press to my 445lbs one took me 5 years and I do know a thing or two about training.
But linear progression is enticing. We can’t resist doing it. I myself have been guildy of making predictions after a spurt of rapid progress. For example I would gain 25lbs on my bench press in 2 weeks and then extrapolate that I would be bench pressing 100lbs more in a few months.
Sadly it doesn’t work like that.
Fat loss is easier to predict since, even though other variables come into play, the caloric deficit is pretty much directly related to the weight you gain or lose.
But with gaining strength there are many aspects to consider:
Strengtrh is dependant on neural efficiency (how good your nervous system is at recruiting muscles fibers, making them work together and making them fire at a high rate). Neural gains tend to be really rapid at first (so you an have very fast gains in strength) but these gains taper off really fast too,
Muscle growth can also increase strength But muscle growth is a much slower process and is dependant on many factors including nutrition, training, supplementation, hormonal status, etc.
Technique on the big basic movements also play a role since proper technique allows you to showcase your strength on the lifts you are doing. If your technique improves your strength will seem to go up since you will be lifting more. On the other hand if your technique is bad you will not be able to showcase your muscle strength as well as you should.
Muscle weaknesses can hold your lifts back until they are corrected. Strenght can go up fairly rapidly and without glitches at first. But at one point you will reach a point where you hit the wall because one muscle or angle might be holding you back. And it might take weeks if not months to properly diagnose the issue and correct the problem.
Trainability is also an issue. For a beginner adding 2% per week will be piece of cake. But an advanced lifter might be happy to end up with a 5% progression from a 10 weeks training cycle… heck at some point you have lifters who will fight for moinths to add 5lbs to their personal best.
I wish I could tell you that you should be able to progress linearly by X% per week, but it just doesn’t depicts reality.