As far as I understood if you buy a mobile tier (one modem for you) you can put as many accs as you want on it. BUT… say you have 10 accs on a modem. And let’s assume 1 or 2 accs of these have bad karma (bad settings, low trust score for whatever reason) the disease of these 2 accs will also spread on the others on the same modem.
With 1:1 mobile proxies the provider makes somehow sure that disease is not spread. Seems to be like a secret sauce. But from what I have read here so far the secret sauce works.
And concerning the “disease” I can tell from my own experience when you have trouble (PV, ban, blocks) on one acc on a mobile tier I always got similar trouble on furthers accs on the same tier (luckily not on all).
Hope this explains it a bit. If not… Sir Henry will be able to tell you in a much better way.