Question: Sometimes I make whipped cream in my ISI cream whipper and it comes out beautifully fluffy and smooth, and other times when I dispense it, the cream looks "ragged" for lack of a better description. Any ideas what factor accounts for the difference?

Answer: 
Four things:

(1) You are definitely over whipping the cream. Please read the instructions - these are tested in the iSi lab to ensure your best results.And they do give you the correct amount of shakes per the fat content of the cream. You describe what we call "choppy" cream.

(2) Hold the whipper upside down in your hands. This means that your little finger is on the lever - not your index finger. This can position is very important because you need to have the gas behind the cream when it's dispensed. this will ensure that the gas is working with gravity and going through the cream and will result in less sputtering, and a full dispensing of your cream.

(3) Make sure you are using iSi branded chargers. Not all chargers are the same, and while the less expensive chargers will fit onto the charging mechanism, it can damage the pin of your iSi whipper and will VOID your warranty. The off brand chargers will provide an inconsistent amount of gas in use, and contain a high amount of oils from manufacturing that will be injected into your foods, this can change the recipe outcome.

(4) Carefully measure your contents, and don't over fill; The head room is very important for the best use of the mixing between the cream and the n20 gas. There is a pour line indicated; make sure your liquid + solid ingredients do not exceed that level (10-x sugar and cream)
I was taught how to whip cream with a handheld mixer and there are a few things to consider

Is the whipped cream being over whipped? When you beat it too hard with a handheld it starts to form ragged peaks

Is the cannister almost empty? Gas charged cannisters often 'sputter' a little bit when over-charged or almost empty.