Comment 10 for bug 121555

The first reason for the second most widespread variant is mnemotechnical (but the one in launchpad is more simple to think out...):

If we define plural forms as:
nplurals=4; plural=(n%100==1 ? 1 : n%100==2 ? 2 : n%100==3 || n%100==4 ? 3 : 0
we get:

msgid "%d file"
msgid_plural "%d files"
msgstr[0] "%d datotek"
msgstr[1] "%d datoteka"
msgstr[2] "%d datoteki"
msgstr[3] "%d datoteke"

or to make it more clear:
0, plural: "5 datotek"
1, singular: "1 datoteka"
2, dual: "2 datoteki"
3, plural for 3 and 4: "3 datoteke"

Where we can see, that the attributes match with the plural number.
So if we are going to unify plural forms it is much better from translators' point of view to have them in this form.