[Proj] problem with Tangent plane stereographic proj

[UW+R SA, J Vallet]

I am encountering problem with a projection (type tangent plane 
stereographic) that does not give the same results if i run the 
projection one time and then the reverse. the lat, long differs 
significantly:

my projection:
lat/long (wgs84)
45d48'05.3757 4d35'31.7851 296.944
45d48'05.3685 4d35'31.8064 297.014
45d48'05.3624 4d35'31.8254 296.924

-> ENU (+proj=stere +ellps=wgs84 +lat_0=45.65 +lon_0=4.55 +units=m +k=1)

3277.74 16838.81    296.944
3278.20 16838.59    297.014
3278.61 16838.40    296.924

->Lat/Long with Inverse projection (-I)

45d40'45.433"N  4d45'58.076"E    296.944
45d40'45.448"N  4d45'58.066"E    297.014
45d40'45.461"N  4d45'58.058"E    296.924

Theoritically, it must be the same as the fisrt set of coords...but not

what am i doing wrong???

thanks in advance for your help!

Julien