What stops them from saying 155m+ then? I still think that xbox is over tracked
155 is far too precise to be an estimate. You could say for example 100+ million or 150+ million or 200+ million, so there's much more wiggle room if they are not certain of the numbers.
Bear in mind AMD has no idea how many GPUs have been used for replacement consoles, Microsoft doesn't give numbers, so the estimate has to have a lot of wiggle room. Example, if you have a console that breaks and you return it for a replacement, it still counts as 1 console sold, not 2. Even tough AMS sold 2 GPUs to Microsoft/Sony.