Paul, those two rightclick context menus are for two different HTML elements. The one with the spill chocker is from the <textarea> where you type text, the other one seems to be for an <iframe>. My guess is that it isn't the amount of text, but rather where in the textbox you rightclick that matters.
Try typing just a single word, and then rightclick on the word itself.
In your two examples you can try clicking the last menuitem, "Inspect Element", and you should see that you end up in two different part of the HTML source.
OK I give up.
I just spent a minute right clicking and found that it seems to go in runs of one menu or the other.
I just right clicked with no text at all this time.
At first it seemed like clicking near the flashing cursor gave the spell checker menu, but I found that once that menu is up, clicking further away gives the same menu.
Eventually it changed when I was half way across the edit box and then as I worked my way back towards the cursor, I kept getting the non-spell checker menu.
Once I had started typing this, I could no-longer get the wrong menu, so I'm not sure about the object inspection.
I have decided that I don't care why its doing it. If you keep clicking in different places, you will eventually get the required menu and then will be able to enable the spell checker. The more text the easier it gets.
I have already spotted several mistakes as I am typing this - so I'm happy (if still confused).
Thanks everyone - Paul