coordinate systems - Proj4js, transfomations forward and back - to big inaccuracy -

I use Proj4js v.2.3.3 to change coordination between two estimates. The definitions of the projections I found on spatialreference.org, so I think it's fine. If I change point coordinates (pstart) from 1 launch to 2 seconds and back to 1 (band) then the pend and paste are very different - about 70 meters in a coordinate. My code looks like this:

  var p3857 = "+ proj = merc + lon_0 = 0 + k = 1 + x_0 = 0 + y_0 = 0 + a = 6378137 + b = 6378137 + towgs84 = 0,0,0,0,0,0,0,0,0 + + units = m + no_defs "; Var p2180 = "+ proj = tmerc + lat_0 = 0 + lon_0 = 19 + k = 0.9 993 + x_0 = 500000 + y_0 = -5300000 + ellps = GRS80 + units = me + no_defs"; Pstart = [2450639.368,6801358.149] P = Prog 4 (p 3857, p 2180, pirate); Pend = proj4 (p2180, p3857, p);  

Next to:

  [2450639.3686302807, 6801287.767802416]  

Did I do something wrong? Can anyone help me in this issue? Thank you all for your interest in the issue