The skill math is pretty simple. I’m not sure why you would find it confusing or subject to interpretation. There are cases (e.g. damage resistance bonuses) where it may not be clear the interpretation of the bonus itself, i.e. what the bonus really means, but the per level skill math, which is what you appear to be asking about, is straightforward in all cases.
If a skill provides say a 5% bonus per level to some attribute, then it does exactly that, provide 5% bonus at L1, 10% bonus at L2, 15% bonus at L3, and so on. There is no other possible interpretation to what the description of the skill bonus says…
Now, this means the attribute in question is modified by a multiplicative factor which in the example above would be:
1.05 at L1
1.10 at L2
1.15 at L3
and so on.
If an attribute is modified by more than one skill, you just apply the multiplicative factor of each skill to compute the result. Since multiplication is associative and commutative, it doesn’t matter in which order you apply those factors.
Thus, an attribute that would be increased by say 15% because of some skill and by say 20% because of some other skill would be modified by a factor:
1.15 x 1.20 = 1.38
which would be a 38% total bonus.