Why do I see two sizes for storage space available?
Having Time Machine active is probably the cause for the difference between the two reported sizes.

For example:

Finder tells me I currently have 55.53GB of freespace on my SSD.

System Information tells me there's 44.22GB.

System Information also tells me 11.31GB are being used for Backups.

44.22GB + 11.31GB = 55.53GB

In Lion, Time Machine will keep Backups on the local drive in addition to the external drive. This is useful for notebooks as you are able to make use of Time Machine without needing to have the external drive with you.

I would guess the oldest backup files get removed once you need the space for something else, which is why Finder doesn't report those backups as taking up any space.