Why are 555 IC pin diagrams so random?
The physical pin-outs of all "555" devices that I know of are the same.

In general, schematic symbols are generated to match the standards and/or prefernces of the organization or engineer that is using the component.  Sometimes, a PCB designer creates the schematiic symbol in any way that seems fit.  Many times a designer will use whatever schematic symbol they can find in a manufacturer or third party library.

Some like to make the symbols match the manufacturer data sheet.  Others like to make the symbols match the physical pinout (so they can visualize a layout while looking  at the schematic or, when debugging, figure out what pin to probe more quickly).  Some will arrange symbols to have inputs on the left and outputs on the right.  Some organizations allow pins on the top and bottom of symbols, some don't, and some only allow power and ground pins.  Some symbols (for digital devices and op-amps etc.) are drawn without power or ground pins; these pins are "globally" connected to the power rails. 

In my opinion, many of these standards do not make sense for many chips.  In my mind, a schematic should show the "scheme" of the circuit; accurately capturing design intent and allowing easy design analysis.

The bottom line is that you may see as many schematic symbols for a part as there are engineers to create the 'perfect" symbol.