[Groff] Is setting the PO register a page trap?

[Dorai Sitaram]

I did a relative set of the PO (page-offset) register at the head of an ms 
document
.nr PO -.25i
(I.e., I wanted it to be a quarter-inch less than its default value.)
Surprisingly to me, even though this statement was placed only once, at the 
beginning of the document, its effect seems to be repeated at every page turn, 
giving smaller and smaller PO's for every successive page. This doesn't seem 
intuitive at all. What gives?
--dorai