Again, what's up with spell checker?

I have vowed today to finish writing the proposal for my qualifying exams. As usual when writing scientific discourses, this has raised several interesting questions about Microsoft Word's spell checking lexicon. As you might imagine from my title, Determining the factors controlling site invasibility to Lepidium latifolium, the word Lepidium occurs quite frequently in this file. Today I decided to add it to the spell check dictionary, after a week and a half of ignoring the red, jaggedy underlines. Much to my amusement, in doing so I discovered what words Microsoft was just itching to replace it with. They (and their definitions) are:

Aecidium - A form of fruit in the cycle of development of the Rusts or Brands, an order of fungi, formerly considered independent plants.

Lixivium - A solution of alkaline salts extracted from wood ashes; hence, any solution obtained by lixiviation.

Residuum - something residual, as a : RESIDUE; b : a residual product (as from the distillation of petroleum)

Of these 3, only the last was freely available in the standard Merriam-Webster Dictionary. The other 2, along with Lepidium itself, you had to consult the unabridged behemoth for (or a different online dictionary =D). Now that I have encountered "Lixivium" and, even better, lixiviation, I plan to incorporate them much more frequently in my daily vocabulary!

So why does Microsoft have such obscurities as Aecidium and Lixivium, but not Lepidium? And perhaps more pertinently, why would it think that I actually mean them when I type Lepidium? I'm surely stumped.