[R] intercept value in lme

[victor]

Dear all,

I've got a problem in fitting multilevel model in lme. I don't know to 
much about that but suspect that something is wrong with my model.

I'm trying to fit:

m1<-lme(X~Y,~1|group,data=data,na.action=na.exclude,method="ML")
m2<-lme(X~Y+Z,~1|group,data=data,na.action=na.exclude,method="ML")

where:
X - dependent var. measured on a scale ranging from -25 to 0
Y - level 1 variable
Z - level 1 variable

In m1 the intercept value is equal -3, in m2 (that is after adding Lev 2 
var.) is equal +16.

What can be wrong with my variables? Is this possible that intercept 
value exceeds scale?

Best regards,

victor