This answer mentions that (optical) spy satellites are often in elliptical orbits, and when I look here sure enough the orbits are elliptical, with periapsis altitudes around 250 to 400 km, and apoapsis altitudes around 400 to 1000 km. I'm guessing it is some kind of compromise involving resolution limit by distance and orbit decay, maybe even atomic oxygen, but why would a satellite with a minimum altitude of 400km still go up to 1000 km every orbit instead of being circular?
This is just an educated guess; I'm not connected with the National Geospatial-Intelligence Agency, or any related agency.
Europe is in the northern hemisphere, as is the Middle East and all of mainland Asia. Those are the primary targets of the United State's satellite spying efforts. What about the Southern Hemisphere? It's mostly water (not a threat), a all of Australia and New Zealand (both allies; neither is a threat), parts of South America (some are allies, none are technical threats), parts of Africa (no technical threats there, either), and a bunch of islands (not threats).
Since the Southern Hemisphere is "mostly harmless", it makes sense to have an elliptical orbit with perigee occurring over the Northern Hemisphere and apogee over the Southern Hemisphere. A 250 km circular orbit would either require a lot of fuel for altitude maintenance or would require frequent launches. An elliptical orbit with the perigee small over potential threats but apogee a bit larger over the "mostly harmless" areas reduces overall drag and extends the life of the satellite.