Here's my guess:
We have one SKU but two different UPC's.
The slipcase and the 360 game case inside both have different UPC numbers.
Ubisoft was only going to produce the slipcase-edition of EM with the graphic novel for a limited amount of time. To save money, they gave the internal case a different UPC so they could just keep making copies and not stick it in a slipcase.
TRU kept the same SKU but changed the UPC. The SKU for the slipcase EM now points to the non-slipcase EM. But the UPC for the slipcase edition is still in the system and still at 59.99.
So if the videogame department updates their dividers, they'll paste the $19.98 price on, even though it's technically for a different product.
Now this raises a bunch of different questions, namely is this an internal screw-up, a future TRU deal that got into the system early, or an official drop by Ubisoft.
So...
If you get the actual copy of EM scanned, its UPC is 59.99.
If the updated UPC on the divider is scanned, it's for the 'new' EM at 19.98.
If you manually enter the SKU it would also be for the 'new' EM at 19.98.
Right now, there only seems to be two ways to get EM at $19.98: Get the cashier to manually enter the SKU by bringing up the display box under the guise of a price check, or if they tagged the divider, bring that up and argue a price change.