Sequence of events:
-Act 1, I had a character (DH) I was playing in single player, and I think I got only to saving Deckard
-Using DH, joined a friend's game temporarily just to chat.
-Started new character (Wiz) with a full party, started from the beginning, and beat the skeleton king before stopping.
-Next day, go back to playing DH in single player. He is same level, but current quest is to find the manor, I can take the ferry, and I have access to many Waypoints than I got even with the group or have ever played to (where monsters beat him to shreds very quickly as he is only level 8).
A possibility is when I joined my friend temporarily just to chat, I may have continued to get his quests completions and WPs when he progressed through the game later even though I was not logged on.