Here is an example of why the kerning chosen between any two letters will always be some sort of "compromise"......The 2 words in the top line is one typeface while the 2 words in the bottom line are another typeface. The kerning for the "sb" letter pair on the top line is looser than the kerning for the same letter pair on the bottom line....But observe what happens as a result of these 2 different kerning "philosophies" for the "sb" letter pair....We have the top left word being almost broken up into 2 pieces from the loose kerning of "sb", while the word at the top right looks nice and evenly spread out......Now the bottom left word with the tighter "sb" kerning does not look broken up into 2 pieces, but the bottom right word does not have a nice and evenly spread out look because of the looser spread of the r and e letters before the "sb" letter pair compared to the tighter "sb" kerning used. Now of course one could have built into the typeface both a tight sb pair and a loose sb pair to choose from, but to wipe out all example of the situation described and illustrated here, one would have to have a huge amount of lettering pairs available in the 2 choices, something that is simply not practical to do. Now many typefaces do have built in lettering pairs, but as an example, I do not think many have the choice of 2 lettering pair kernings for the "sb" lettering pair for example. As a result of all of this together, some sort of compromise must be allowed regarding the kerning of letter pairs if one is to avoid a huge slew of different lettering pairs choices having to be built into the typeface.