How can the replacement level fertility be 2.1 everywhere in the world? Common sense would indicate that the replacement level fertility of Norway ought to be lower than that of Sudan due to fewer children dying there. Why, then, is the replacement fertility rate universally taken as 2.1?
Your claim that the replacement level fertility is "universally taken as 2.1" is false.

World Resources Institute:


  This rate is roughly 2.1 children per woman for most countries, although it may modestly vary with mortality rates.


Craig (1994):


  In developed countries, replacement level fertility can be taken as requiring an average of 2.1 children per woman. In countries with high infant and child mortality rates, however, the average number of births may need to be much higher.


2.1 is simply a very crude and made-up figure. It's just 2 (replace the parents) plus a little more (arbitrarily assumed to be 0.1) to account for mortality.

Where a writer is unable or unwilling to obtain better estimates, she will often simply use this 2.1 figure. But this does not mean she believes that the true replacement level fertility in her context is precisely 2.1.