What is the point of this line in the Hide of the Feral Guardian?

The Hide of the Feral Guardian, a legendary item from the Explorer’s Guide to Wildemount, includes the following ability.

When you cast the polymorph spell using this armor, you can transform into a cave bear (use the polar bear statistics).

Why does it do this, and not just turn the user into either a polar bear or cave bear directly? At first I thought it might be because of the book each creature was from, but they both have stat blocks in the monster manual, and neither are in the explorer’s guide elsewhere as far as I can see.