SN74LVC1G125: Why is there two current values for IOL and IOH at 3V?

They correspond to the two V_OL/V_OH entries in the electrical characteristics.

This is true. But as for why we give two different test currents for a single VCC voltage at some VOH, there isn't really a good reason. Maybe some competitor had it before us and it didn't cost us any money to copy them, so we did. But there's not an underlying explanation I'm aware of that is otherwise significant or useful.