In its centennial year, for what- ever reason, the Washington Monument attracted 12 percent fewer visitors to its top in the first seven months of this year, compared with the same period last year. The White House reported a decline of about 10 percent in the number of touring visitors -- and, this being a political year and a nonpolitical column, we'll shun speculation over why.

On the other hand, both the Lincoln and Jefferson memorials showed increases in tourism. Here we'll speculate about one reason: both have more plentiful parking plus year-around visiting hours until midnight.

And, the National Park Service reported, the Frederick Douglass Home in Anacostia, one of the shrines of black Americans, also showed a significant proportional increase in visiting although, in absolute numbers, the number of visitors was small. The increase is due, one suspects, to the laudable decision by the Tourmobile people to add the Douglass Home to their itinerary.

Here are the figures for the first seven months of the two years:

Washington Monument, 1984: 715,190; 1983, 803,571.

Lincoln Memorial, 1984: 2,099,443; 1983, 2,019,765.

Jefferson Memorial, 1984, 1,184,127; 1983, 1,029,615.

White House, 1984, 627,618; 1983, 699,923.

Douglass Home, 1984, 22,035; 1983, 19,627.

Visiting hours at the Washington Monument, in a routine seasonal move, were cut back yesterday to 9 a.m. to 5 p.m. from a longer summer schedule. Hours at the Douglass Home likewise were cut back to 9 a.m. to 4 p.m. Hours at the Lincoln and Jefferson memorials continue to be 8 a.m. until midnight, and at the White House from 10 a.m. to noon Tuesday through Saturday.