You are making it look like bounties including more than just the strength of the individual means their value is reduced, as if bounties focusing on strength alone would be higher than they already are; that makes no sense. Just imagine it: "This guy's strength alone would grant him two to three billions, but since he's also a vicious pirate and Kaidou's right-hand man we will increase it —no, no, what am I saying, we will LOWER it to 1.39b". Absurd reasoning.
King wouldn't get a higher bounty if the Government focused on his power level alone; odds are his current bounty already contemplates his strength peak. In other words, he would get a lower one if only for his strength. It's not a coincidence that the most established bounties in the world, which are those of Roger-Whitebeard-Kaidou-Big Mom (because two are dead and the other are done, so we're granted they reflect their threat level with no future variations), perfectly fit the strength hierarchy stressed throughout the story:
1) Roger the Pirate King, ergo on top of every conqueror.
2) Whitebeard the World's Strongest Man, the World's Strongest Pirate, the only one known to equal Roger, the one highlighted by Linlin in comparison to Shanks and Kaidou.
3) Kaidou the World's Strongest Creature consistently addressed as the current strongest.
4) Big Mom the weak link of the four.
King's value is 1.39b. If we focused on his strength alone, it would be significantly lower (no Kaidou's influence, no vicious behaviour, arguably no lunarian extra). In fact, I highly doubt any character but a very few (emperors, Mihawk, etc.) would even reach 1b based on their strength alone.