machine numbers are rarely accurate... go by intensity and time, use those numbers as an estimate (i read that some machines over estimate by up to 30+%) and i can personally tell the tension other little differences between different machines of the same make.. this means theres even inaccuracies based on whether you use the trainer in front of the tv (which is used more) or that one in the corner which is never used except by 500lb ladies who are trying to be inconspicuious (sp?)
but... as a numbers oriented person, personally i always put my weight into the thing at about 20-30% less than it is, that way at least i compensate for the first source of error (and try to use the same machine all the time)...
(remember, any machine where you enter your bodyweight does estimates based on that (bikes do not, some treadmills do not) and the machine doesn't know your body composition or metabolism so it just averages (probably using an out of shape individual, thus the higher numbers)... machines that do not have that feature estimate based on other factors, and, for most people, they are even less accurate than the others...)