The reach curves and media viewing habits reflect what proportion of the population you can reach with a given Advertising or Public Relations (PR) mix. These are the same for both Advertising and PR partly because it is reasonable that the advertising reach and viewing habits should be similar for both product advertising and PR.
The sensitivity to Advertising and PR reflects how much each market segment is influenced by any Advertising or PR they are exposed to. If a segment has low sensitivity to PR, then even if they are exposed to a lot of it, there will be a much smaller influence than if they are highly sensitive to PR.
If a segment is highly sensitive to Advertising, then there is probably going to be a reasonable market share difference, but if there was low sensitivity to PR then you would probably only see a small market share difference (ie. you are probably wasting a lot of your PR spend and you should look at more effective ways of using that money).
The market segments have different sensitivities, so you need to find balance between Advertising Awareness and PR in each market.