1) Wins initially, and keeps winning.
2) Wins initially, but then reverses to become a loss.
3) Loses initially, and keeps losing.
4) Loses initially, but then reverses to become a win.
If you average down, you only get the chance to add to trade types 2, 3 and 4.
If you average up, you only get the chance to add to trade types 1, 2 and 4.
Averaging down tends to be attractive to people, since it allows the possibility of trade type 4 i.e. a trade that goes against you, but then reverses to recover your losses and more. However, that comes at the material risk of trade type 3 i.e. the trade that never recovers.
Averaging down virtually guarantees that your biggest positions will be in trade type 3 i.e. the trades that never win. Pyramiding up avoids this risk, and also allows you to add to trade type 1 i.e. the trades that start off winning and keep winning.
To be clear, there is a legitimate strategy of picking a range of entry into a trade. Rather than picking a particular price point, you may choose to scale into a position over a range of entries. The distinction here is that you must decide this plan before the first entry is made, rather than in response to a trade going against you.