I also mixed Longitude and Latitude until I heard someone saying '... at high latitudes.'.
In my mind, the picture of the Earth shows the Earth with the North and South poles at the top and bottom (or reversed, doesn't matter). High describes a vertical orientation. Therefore, the latitude changes in North-South direction.
Additionally, because the horizontal degree scale has a discontinuity at 360 degree (0 degree), it does not make much sense to distinguish high and low values in horizontal direction.