You may remember from yesterday that the case forms of год are:

Today we need to discuss the genitive singular forms. The modern version of the genitive singular is года, but there is an older form that arose as a consequence of the u-stem genitives which nowadays only appears in stock phrases. So you will occasionally find phrases like:

For instance:

There is a tendency in language to make the things that are irregular more regular. Thus the older plural ‘brethren’ in English has been mostly replaced by ‘brothers.’ Similarly nowadays it is much more common in Russian to say «год от года» than «год от году». It is still more common to find «без году неделя» than «без года неделя», but that ratio will most likely eventually change in favor of года.

Language changes. For instance, as I was growing up, I never heard anyone say “You want to come with?” for “Do you want to come with us?” I remember the first time I heard it. I thought “No native speaker of English could possibly ever say that!” I was wrong. All sorts of US native speakers say it nowadays, and nowadays it almost doesn't sound strange to me. This should teach us a lesson. No native speaker ever knows the entirety of his language. And if we find that Russian speakers occasionally give us contradictory language about their native language, this is also normal. In such situations, you need to ask multiple native speakers about your language issue, and then draw a rule of thumb from their responses.