The reasons that the resistors are included is because the output from the Atari is at a higher voltage level than the input the monitor or TV is expecting.
Some can cope or have controls that you can adjust. But with some it "over drives" the monitor or TV circuits and you end up with too bright an image.
There is an additional reason if you are using the monochrome high-res output to drive a colour monitor, in that for the colour you want, the signal has to be split in different amounts to go to the three colour input channels (RGB) to give you the correct colour.
Through experimenting with different values people have found the "right" values for their monitor or TV. That is why there are different values used.
Hope this helps
Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (nearly forgot the Psion's)