Confusing Latin

I have learned that common names can be used for multiple plants (Pigweed is one and I think we have the three listed on Wikipedia!!!) , so when writing about something, I try to use the Latin names. But to really confuse things, I am finding some plants have multiple Latin names, such as this plant.
http://davesgarden.com/guides/pf/go/56502/

Why is this?

You might find this helpful.
http://tomclothier.hort.net/page46.html

Latin names get updated. The first botanist to describe a plant gets to name it, so when some old botanical records are found, a name can be contested and changed. However, if people refuse to recognize and use the new name, occasionally it gets changed back. Also, now they look at DNA, families can get split in two or combined.

So in theory binomial nomenclature is more accurate, but like you, I have found it has flaws. And it isn't actually "Latin". Like the spells in Harry Potter, it is a combination of Latin, Greek, old English (or old Botanist) and Whimsy. If the name includes a person's name, it isn't always the person who discovered it - Botonists loved to name things after each other.

Cactus are a bit of a hobby of mine, and the names are very inconsistent. It is more or less "pick your favorite expert, then use their preferred names".