One symbol, another symbol
Since antiquity, when spoken language was established but not written, those leaders (kings, priests, etc.) used symbols as visual expressions of power and for communication.
Circles came to represent wholeness, completeness of a cycle.
Crosses came to represent experience, physical things, matter.
Cressents came to represent our collection process or receiving of spirit.
Different cultures and religions refined these and other meanings over time. The planets were seen to be gods, and their symbols had to represent their attributes.
The Earth became a circle with a cross within it -- an unfolding cycle and completeness of living in the material or physical world.
The Sun became a circle with a dot within it -- a focus on the wholeness of all that is, of the universal mind.
Placing various elements above and below, or in combination with any given symbol supposedly defines the "god's" meaning. Over time the combination and writing/painting//sketching of the symbols became a little distorted. Look at Saturn's symbol and Jupiter's symbol. Originally they were the same expect being reversed. Today's symbols don't easily or clearly show that.
As for Pluto's two symbols, well, the "P"-like symbol is easier to write. I am on vacation and don't have book references to precisely define the symbols. Others can quote you what the books say. What is important, today, is not what the symbol looks like but what it means to us. Dave
K