Okay, I am getting frustrated. Ha ha, not really. But can somebody explain this phenomenon to me?

One ruble = один рубль
Two rubles = два рубля
Five rubles = пять рублей
Twenty-one rubles = двадцать один рубль

One year = один год
Two years = два года
Five years = пять лет
Twenty-one years = двадцать один год

Okay, the рублей doesn't bother me too much. It's just different endings of the same word. But why года/лет? Two totally different words!

What other words work this way?