500 charge limit on wireless charger for Nexus 4 / 5 (QI)

I recently bought a QI compatible charger for my Nexus 5. On the box it states, that the **"wireless charges up to 500 times"**. - I don't understand why the charger is only able to charge my phone 500 times. - What happens to the charger so that it does not work anymore? Can someone explain that? Thanks for you help. It is exactly this device: QI charger from ravpower **Edit 2:** Looking at the website of the maker, it seems that they sell a mobile power bank as well. Perhaps they use the same packaging for the mobile power bank wireless charger and the wireless charger without battery. **Edit 3:** I found the packaging on the internet.