Here's a pretty good definition of a false flat; take note of the last paragraph.
http://www.fitsugar.com/What-False-Flat-8216271
"You know that sensation when you're on a stretch of road, either running or cycling, and it looks flat but you're still huffing and puffing? When you look at the road or trail straight on there is no perceptible rise in the terrain, yet you're working extra hard to maintain your speed? That, my dear, is a false flat, and the roads in my neck of the woods are full of them. The slow creep of roads away from the San Francisco Bay offer many almost imperceptible uphill grades, but I have felt them elsewhere. False flats are deceptive and can be soul crushing; unlike discernible hills, there is no victorious summit creating a sense of accomplishment and a bit of glorious descent. One biking forum explained that the grade of these ascents as only being one to two percent, but that is enough to feel it on the climb up.
Coming down a false flat on the other hand makes you feel quite speedy, but rarely will one attribute their sudden speed to the downhill false flat. The term seems mostly used to explain the difficult, if imperceptible climbs."