Is there any lore reasons for stuff like this?

1. Geralt getting weaker at the start of each game, at the end of TW1 and 2 you could destroy nekkers and drowners in 1 hit, so why at the start of TW3 do Drowners nearly destroy me?

2. Potions acting differently in each game

3. signs acting differently

4. Geralt forgetting how to make certain potions/bombs?, pretty silly losing my ability to make all the bombs and stuff i learnt how to make in TW2

New gameplay mechanics. No lore explanation. I never could get that too. Even Mass Effect, apparent inspirational source for W3, allowed certain amount of skillpoints being transferred.
Besides, new skill system is hilarious - to, say, apply an oil that does not degrade, you have to give up some skill, occupying in one of slots. /grin

Geralt has become really forgetful as he ages...

Yeah, I don't like how slow progression and non-lore it is for him at the beginning either.