These are general guidelines. The reasoning is that on a 'dry' board you have a range and NUT advantage as the preflop raiser against the BB caller. As the BB should 3 Bet most of his good overpairs and his bets two broad way cards. This means that you have so many good hands that your bluff frequency is not that high that you violate rule 2 if you follow rule 1. On dry boards even AKo is ahead of most of villains range and thus is a 'value' bet.

Ok this is basically true but you have to understand that bluffs/value aren't so strictly defined. Bluffs can expect to become value sometimes and value can become not so value. All that matter with the strength of a hand is the strength on the river. Also a little more advanced of a concept is that all your river bluffs actually act as value on the turn because they are 0EV to bluff on the river. The same applies on the flop where every hand you are betting the turn will act as value on the flop. This is some advanced theory but it basically means that say you bet 1/2 pot on the flop and think you should have 75% value because your opponent get 3:1. In reality you should have maybe 25% value (maybe even less although I can't do the math). This is why so much of the population drastically underbluffs.

A small range bet is not a 1/2 pot bet in my book it is 1/3 pot or less. So you do not need 75% of value hands and if the bet is smaller what can be considered value is also lower.