According TFA the number of people in extreme poverty dropped when using the old IPL value, and went up with the new value.
So politically, no NGO wants to say poverty decreased, because that might reduce urgency, and thus priority. So moving the goalposts means a 50% increase in poverty instead of a 20% decrease in poverty. Which one benefits your mission more?
That's not to say the revision of the IPL was wrong. But it does further the mission. Did the improved statistical methods trigger the IPL revision? It's hard to tell without internal world bank docs. I'll bet it did.