Maya python API: How to make float array attribute editable in the channel box/attribute editor?

Question

I'm writing a custom node and I need an array of floats. I noticed that if I create an array attribute with om.MFnNumericData.k3Float type, then I can add new elements and edit the values in the attribute editor:

But if I change the type to om.MFnNumericData.kFloat, I can no longer do that:

Is this expected behaviour? What should I do to have the float array attribute editable?