That isn't so surprising nor uncommon even among high-end chargers. Many multiple-port chargers can only generate one voltage other than 5V. So, you charge you

That isn't so surprising nor uncommon even among high-end chargers. Many multiple-port chargers can only generate one voltage other than 5V. So, you charge your laptop at 100W means 20V*5A, then connect a phone that can only negotiate to 9V. Now your laptop can, at best, only get 45W. But laptop and phone might also have to share the same 5A, meaning something close to what you described. (or, your laptop might not support 9V ... in which case I'd guess one of the devices would disconnect?) You might be better off using a USB-A port if the charger has one, then your laptop could perhaps remain at ~100W and your phone might still get ~10W. (Assuming charger and phone doesn't also have quickcharge which might complicate it further). Or, perhaps charge your phone via your laptop (yes, negates the point of a multiple-port charger, but if you are in hurry - otherwise, slow charging is preferably anyway) A good review will test this, and a good product page will specify this (but if your are not looking/thinking about it it is easy to miss).