The two ratios are not separate. If you have two fractions then the fractions are multiples of each other. They will both reduce to the same numerator and denominator.
For example .
Usually when you are doing proportions you are given 3 things right? and you have to find one.
So say you have , you want to know what to put in place of a so that becomes .
Observe, the algebra is not just nonense, so multiply both sides by 5, you get .
Now do bring the 5 down, and you get knock out the 5 in num and 5 in denom, you get , just what we wanted out of the equality.
Also the problem lies with using the = sign, when infact they are equivalent .
Meaning, is equivalent to , you get what i'm saying? is equal to only
Also i admire you for asking such questions, they are of fundamental importance, nothing to be trivialized. Keep thinking this over (Asking questions like what does it mean for 2 things to be equivalent rather than equal ).