We cannot really explain tipping in terms of how much work the server does. Tipping is not something that is done primarily in order to reward the amount of effort put into serving you.
Economists such as the author of this paper have studied tipping to try to find out why people tip and why they tip certain amounts. They have determined that most people make their decisions based on social norms. What that means is that people tip because they are supposed to. They worry that if they fail to tip others will think less of them. The tippers are giving a certain percentage of a tip not because they think that is how much the server's effort was worth but rather because they think that is how much they need to tip in order to look good in social terms.
So, in the example you ask about, people tip different total amounts for the same effort because that is what is expected of them. They do not want to suffer social sanctions (having people look down on them or think they are stingy) and therefore they tip the accepted amount regardless of whether that makes logical sense.