You should see this from the old versions viewpoint, where wehad the old training system where we saw the gaining progress in each training;
To win 1%, you need to complete a full bar -now invisible- of 99% progress
Every training, increase the bar of each skill you train, (passing, shooting, a new position or SA)
"in a different % each time you train", time ago when we did not train each atribute individually, we completed the bar, and we obtained a "cone", that we where able to place in one of the 3 old groups of attributes (attack, def, phis and mental)... sometimes, a HARD TRAINING, (we had 3 types only of training, each one using more condition) gave for example in a young fast trainer, +148 of progress, so one bar and a half.
Now, is the same, but you can not see that every training, in this "hidden bar", make you win a different progress, so maybe you win now +110, then 98, then 105, and then +89.
Thats why the training of a SA "don't give you exactly a +1 in each training" because the progress won, (progress needed to complete a 99% progress bar, wich gives u a +1% in the SA or another) is different every time you do a training session".