"Ratio" in English has two meanings, which in Italian can be rendered as "rapporto" in one case, and "proporzione", which I think is translated exactly by the English word "proportion".
The first case, rapporto, or quoziente, is expressed with / or something to that effect.
If I say that 4/5 of APUG forum participants do not write in proper unambiguous English ;) the second member, the divisor, is the "total"* of the two groups of APUG participants, and the first, the dividend, is those for each five participants who cannot use unambiguous English.
If I say that the proportion between those who can use unambiguous English and those who cannot is 1:4, I say the same thing expressed as a proportion.
Those two kinds of notation, 1:4 and 1/5 or 4:1 and 4/5 are, if I get it right, both termed "ratio" in English.
This is made worse by the fact that / and : are alternative signs for "division", and although it is customary to use / for "rapporto" and : for "proportion" this subtleties can go lost for writers of instruction booklets.
A trained person always reads 1:10 as "one to ten" (eleven parts in total) but many would read it as 1/10. It would be called a "ratio" in both cases correctly but the dilution would be different.
I guess in Italian and in other languages the ambiguity would be resolved because it would be specified "a proportion of 3:10" or "un quoziente di 3/13". In English you are pretty much stack with "ratio" which is "irrational" a way to express oneself ;) (quotient, proportion should find larger use).
* OK it's not the "total" you get what I mean...