What is the reason of the disparity between single article price and yearly subscription in scientific journals? I'd like to understand scientific journals' pricing model. 

For example, in the case of Nature, one single article bought as a unit costs $32.00. But a personal subscription is only $199.00 for 1 Year (51 issues, Print and Online). That is 6x digital articles cost slightly less than all articles in one year and 51 paper magazines. 

What is the reason of the disparity between the article price and the yearly subscription? Wouldn't they obtain a better return rate if they priced the articles at a reasonable amount? (maybe $4-$5)
On the principle a subscription is always better than individual sales from the journal's point of view because it's a fixed income on which they can count. It's also well known to increase sales that explain why you always get a discount when you subscribe to a service.

As for the seemingly large difference you mention, my guess is that individual subscriptions for scientific articles are rare, and thus not very interesting financially. People needing frequent access to the content of these journals typically access them via institutional subscriptions.