Hey Merimen!
I think the confusion comes from this:
If I understand it, pp100 should reflect field % plus some blocked shots and free throws come to the formula.
This is not always the case actually because pp100 is a theoretical value. Citing Justin : "PP100 is the average amount of points the team would score from that position if they took 100 shots". So pp100 is an expectation not related to what really happened in this one simulation of the game and you could observe such a discrepancy between pp100 and field % due to bad luck, just like you could flip a coin 20 times and get heads 5 times.
An interesting thread on this topic:
(254457.1). Hope this helps!