Why does the calculator return 1 for (-1^.5)^2? I would have said it should return -1.

Because ^ takes priority over - we have -1^0.5 → -(1^0.5) → -1 

Square this and you get +1

.

You can 'force' it by entering it thusly   ((-1)^.5 )^2  = -1

Actually, in this case

$(-1^.5)^2$

Is the same as

$\left ( \sqrt{-1} \right )^2 \\ \text {is the same as } \sqrt{-1}= i \rightarrow i^2 = -1\\$