It can be inconsistent for sure. I see the most inconsistency when "shipped and sold" by a third-party. FBA ("fulfilled by Amazon"), or whatever the term is for

It can be inconsistent for sure. I see the most inconsistency when "shipped and sold" by a third-party. FBA ("fulfilled by Amazon"), or whatever the term is for when Amazon stocks and ships it, seems to more consistently meet the 2-day time limit. When it doesn't, the blame can often be placed on the shipping company. Even when the blame is on Amazon, it's a day or two delay, and accompanied by an email with a (usually) accurate reset of expectations.