Good morning, 8runo,
I think I have it.
The only way we can be in position vs a 3Bet is if the 3Bet comes from the blinds.
If we're in CO (or earlier) and someone in later position 3Bets, we're OOP from then on.
If they're in the blinds, though, then we're IP vs them.
This can only happen if the 3Bet comes from the blinds -- the other earlier positions have folded, or made the initial raise. If they made the initial raise, then we are the ones 3Betting, and they're OOP vs us, and their stats will thus fall in the IP column -- we can never 4Bet in this situation, because we 3Bet.
Like you it took me a bit to sort this out.
From EP to BTN you can only be IP vs the 3Bet if the 3Bet comes from behind you, and the only positions behind you that can 3Bet are the blinds.
Here's another way to look at it:
1-Bet (blinds and limpers)
2-Bet (open raise + all calls to the open raise )
3-Bet (1st re-raise)
4-Bet (re-re-raise)
You can see that you can only make even or odd bets.
The only players that can 3Bet OOP are the 1-Betters.
For everyone else, they can 3Bet vs an open raise behind them -- they're then IP.
Whew
Cheers,
VS