The 20% RE does NOT work like this:

RE 1 item = 20% chance; therefore RE 5 items = 100% chance! I r so smart

Rather, it works like THIS:

Will RE X items:

Item 1 has 20% chance of success, 80% chance to fail

Item 2 has 20% chance of success, 80% chance to fail

Item 3 has 20% chance of success, 80% chance to fail

...

Rinse and repeat until you hit the 20% chance.

In short: The chances of success are rated individually, not cumulative.