[Proj] Lat/long to utm easting error

[Adam Dershowitz]

I have run into something that I don't understand, and I hope someone can help explain.
I was doing some conversions of points, and ran into a problem.  I finally narrowed it down to the following:

If I take two points, at the identical longitude, with a small change in latitude, and convert them each to UTM, I would expect nearly identical easting values.  But I am not seeing that.  For example:

cs2cs +proj=latlong +datum=WGS84 +to +proj=utm +datum=WGS84 +zone=17 -r

42d37'3.77503"N 83d59'35"W
254524.57	4722707.58 0.00
42d37'0.02186"N 83d59'35"W
254520.47	4722591.80 0.00


Above there is  ~4 m easting difference, with just a small (~120m) change in latitude.  Since these points are exactly North-south of each other, shouldn't they have the same easting, at least when they are this close to each other in latitude?  

Thanks much for any explanations, 

--Adam