The Level Of The Stats...
The bonuses that you recieved is determined by the max stat that you can receive in the table above. For example a D rank Tiger can only receive 1 STR. That means that it is Level 1. A "B rank Tiger" can recieve 1 to 4 STR points. Looking at the table below, that means that your bonus will come randomly from levels 1 to 3. Because the "4 STR" lies under "Level 3" and the "1 STR" lies in Level 1.
Confusing right? If you have a rank A lion, with the levels 1/3/2/6, then according to the table below, it will have 1+4+2+16 = 23 sta. So in order to have a +75 sta pet, you would need to have an S grade lion, level 1/3/5/7/9. This equals 1+4+11+22+37 = 75 sta. I hope that makes sense
Level 1/ Level 2/ Level 3/ Level 4/ Level 5/ Level 6/ Level 7/ Level 8/ Level 9
STR/ 1/ 2/ 4/ 7/ 11/ 16/ 22/ 29/ 37
STA/ 1/ 2/ 4/ 7/ 11/ 16/ 22/ 29/ 37
DEX/ 1/ 2/ 4/ 7/ 11/ 16/ 22/ 29/ 37
INT/ 1/ 2/ 4/ 7/ 11/ 16/ 22/ 29/ 37
ATK/ 5/ 10/ 20/ 35/ 55/ 80/ 110/ 145/ 185
DEF/ 4/ 8/ 16/ 28/ 44/ 64/ 88/ 116/ 148
HP/ 91/ 182/ 364/ 637/ 1001/ 1456/ 2002/ 2639/ 3367
The Chances Of Each Level...
As you can see from the table below, the higher rank your pet is, the lower the chances of getting a better bonus. So many people who raise a pet to "S Rank" probably didn't get the full potential of that pet.
Level 1/ Level 2/ Level 3/ Level 4/ Level 5/ Level 6/ Level 7/ Level 8/ Level 9
D/ 100%/ 0/ 0/ 0/ 0/ 0/ 0/ 0/ 0
C/ 51%/ 31%/ 18%/ 0/ 0/ 0/ 0/ 0/ 0
B/ 40%/ 28%/ 18%/ 10%/ 4%/ 0/ 0/ 0/ 0
A/ 35%/ 23%/ 17%/ 12%/ 7%/ 4%/ 2%/ 0/ 0
S/ 27%/ 21%/ 17%/ 13%/ 9%/ 6%/ 4%/ 2%/ 1%
As you can see, for your Rank S pet to get the Level 9 bonus, you have a 1% chance. So even if you got your pet there, you actually have a better chance of only getting a level 1 bonus, which only gives you the lowest bonus of them all.