The dew point is a measure of how cold the air would have to get right now before water started spontaneously condensing out of it - or how cold a piece of equipment brought into the room would have to be in order to condense water on its surface.
Naturally, having water on things is a bad idea in a server room, so you don't want the dew point to get too close to ambient (ie, the RH wants to stay fairly low), and you don't want to bring cold equipment into a server room with a fairly high dew point.
Wikipedia's article on the dew point suggests that for 17C and 40%RH, the dew point is about 3C, so I think your meter's probably telling the truth. And from the above, I think that's a perfectly fine dew point as well, so don't worry.