William,
 The routine is working correctly. 18N/80W is outside of the RUC2 domain due to the map projection. Even though the SW and SE corners of the RUC2 domain have latitude less than 18N, on the Lambert conformal grid, the latitude of the southern boundary is about 20N at the center of the grid (near 95 W). If you plot latitude on the RUC / Lambert grid, the lines are curved downward in the middle and upward on the sides of the domain, giving this effect. Hope this helps.
Stan Benjamin