You're missing the point. Yes, some people could work out averages in their head (I can as well) but the vast majority of people couldn't - we're talking about a time when most people left school at 16 and having even a couple of O levels was relatively rare.
I remember adverts about 30 years...
Ha. A couple of years ago, I bought two items at 75p and I had trouble paying for them as "the till was broken and someone else was using the calculator". So, yes, people would have had problems in working out goal averages in their head.
Back then, teams on equal points were separated by goal average, not difference. It was scrapped in the 70s because, as you say, average favoured teams that scored fewer goals.
Bomber Brown scored a shedload of goals for Huddersfield when they won the 1st Division three years running - I'd be staggered if he didn't score a hat trick