It appears to me that your source had small slip in assuming that pipes ID is constant and OD can vary. Please look in to the dimensions of ANSI B 36.10 pipes : a 1'' NB pipe will have 33.4mm. A 2" NB pipe has 60.3 mm OD and a 3" 88.9 OD. But ID of these pipes will be radically different if you consider schedule 5 or sch10 or sch 20 for these fixed ODs . The usage is : tube will have precise OD, with very narrow tolerance , variable thickness and ID . Pipes also, but for pipes OD is nominal , will have generous tolerance. For tubes also there is a tolerance on OD but that is very small, 0nly 0.5%, some times even as small as plus or minus 0.13 depending on specification.In Industry we use pipes and tubes also as structural members, there is no fixed practice of using tubes only.

Best regards

Venugopal