If your IRM was 85kg and now is 95kg I think you could class that as gaining strength, a for instance on one day I could do "x-kg" for 2x6 on dumbell curl on another I do 1kg less for the same reps and exercise , have I lost strength? No! My arms are just not up to it that workout. which unless my head is in the clouds as it sometimes is, is the reverse of what your saying.
It may just mean that on that day you may of "been up for it more".
IMO you shouldnt be doing 4 main sets at your age anyway, enjoy your youth you will get bigger worries when your hormones kick in
If this seems sh1te soz but I have difficulty expressing myself verbally, my mother dropped me on my head.:p