Just reading the [datasheet of the attiny13][1] it says that it can hold its data for 20 years at 85 degrees Celsius and 100 years at 25 degrees Celsius. 

 - Is this irrespective of the read and writes on the micro for example I turn it off and keep it at a constant temperature of 85 degrees Celsius for 20 years and it will loose its data?  
 - How can it "loose" its data? I fail to see this concept. 
 - Also what is with the unit ppm(parts per million?) its mentioned on page 6 in the data retention section but don't understand what its talking about. I've seen it regularly when talking about crystal oscillators but can't understand why its used. 

 

  [1]: http://www.atmel.com/dyn/resources/prod_documents/doc2535.pdf