AI difference between FDA and EMA, HC and TGA

EMA, Health Canada and TGA has revised the AI limit for N-Nitroso Dorzolamide Impurity from 100 ng/day to 1500 ng/day based on the negative bacterial reverse mutation test.

But still FDA has kept AI as 100 ng/day for N-Nitroso Dorzolamide.

Any specific reason for keeping AI as 100 ng/day by FDA ?.
Or when we will expect for revised AI 1500 ng/day for said NDSRI from FDA.