HP Prime: Unexpected "poly1" after cross product

[RickyFlips]

What does it mean when HP Prime prepends "poly1" to the vector/matrix output of a calculation?

I calculated the following:

CROSS([-0.01 0 0],[60 0 0]) - 0.02^2 * 60 + CROSS([0 0 -0.02],[0.5 0 0])

Which returns just the following, as expected: [-0.62 -0.024]

However, when I copy this output to the entry field (I forget what HP calls this) to do a second calculation, my HP Prime prepends "poly1," to it, so I get:

poly1[-0.62 -0.024]

I haven't been able to figure out what this means or why it shows up, only that I have to manually delete it before doing the next calculation to get the expected answer.

Thank you!