I've seen that once, and it kept me from buying them at that time. 2.5" Borlas that definitely got smaller inside the case. In that situation i was told (from the counter guy) that it was indeed 2.5" throughout, but i know what i saw. However, that was once, with one model (forget which, it was a long time ago... but likely just a basic common model). Most i've seen since have been what they advertise.
I love Borlas, and for me, i'll only put Borlas or Spintechs on my cars. My advice would be to get your hands on them before you buy them, if you dont like what you see, dont buy 'em.