I've met some of those extra liches in the unmodded game (like in the Shade Temple if I go there later than normal). The enemies you find are definitely tied to your experience (not sure if it looks for highest value in party, just the PC, or average value--but all these are the same when soloing). So since you have such a high exp value from soloing, you are finding the tougher baddies.
Also, as others have mentioned, some mods could have added more monsters, but in your case, it's probably just the exp.
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"Many are my names in many countries. Mithrandir among the Elves, Tharkun to the Dwarves; Olorin I was in my youth in the West that is forgotten, in the South Incanus, in the North Gandalf; to the East I go not"
--The Two Towers
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