Your Trash (that icon on your Dock) is a curious beast. It opens just like it's a folder, but it isn't, not really. It's actually a collection of folders whose contents are logically merged to be displayed as if they were all in a single folder.
It has been this way since the very first Macintosh, back in 1984. The issue is that when you want to trash something, it needs to be moved to the Trash, but files cannot be moved from one disk volume to another. The only way to handle this is to create a separate Trash folder on every disk volume. (They're created as needed, the first time you trash something from that volume.)
With OS X and multiple users, it becomes important for each user to have their own separate Trash. That means there is a trash folder for each user on each disk volume.
Your Trash folder on your home disk volume (the disk volume, normally the boot volume, where your home folder lives) is ~/.Trash. On each other disk volume, your Trash folder is at <path to disk>/.Trashes/userid.
The GUI is trying very hard to make all these folders appear to be a single Trash folder. So, what name should this single virtual folder have? Why, it should be called "Trash", of course.
When Disk Utility is showing you the path to a disk image, if the actual image file is in any of your Trash folders, Disk Utility simply tells you it's in /Trash. It's trying to maintain the illusion that you have only a single Trash, not a slew of trash folders scattered across multiple volumes.
Finder is the main instigator in this deception, showing all the folders as a single folder, but relents somewhat if you Get Info on one of the items in your Trash. If it's in ~/.Trash (and your home folder is in its usual place), Finder says it's in BootDiskName ▸ Users ▸ username ▸ Trash. (Notice how it hid the period in .Trash—you aren't meant to see names beginning with period.) If it's in one of the trash folders on another disk, it says DiskName ▸ .Trashes ▸ Trash. (This time it shows the period in .Trashes, but hides the userid, and invents the final Trash out of nowhere.) Also notice that Finder is using ▸ as the path separator, which is probably just as well considering that these are not actual POSIX file paths.
All of these "paths to Trash" end the same, with Trash. The message to the user is: there is only one Trash, even though there may be multiple paths to it.
